What is a "spellcaster" for the purpose of magic item attunement?

Question

Several wands and other magic items in the DMG cite "spellcaster" as an attunement requirement, specifically "requires attunement by a spellcaster" while others list multiple spellcasting classes (e.g., a Staff of Fire "requires attunement by a druid, sorcerer, warlock, or wizard" p. 201). However, all the DMG states in this regard is:

Some magic items require a creature to form a bond with them before their magical properties can be used. This bond is called attunement, and certain items have a prerequisite for it. If the prerequisite is a class, a creature must be a member of that class to attune to the item.¹ (If the class is a spellcasting class, a monster qualifies if that monster has spell slots and uses that class's spell list. [emphasis and footnote added] (p. 136)

The language of the Mage Slayer feat describes a spellcaster as a creature casting or concentrating on a spell.

You have practiced techniques useful in melee combat against spellcasters [...] When a creature within 5 feet of you casts a spell ... [and] When you damage a creature that is concentrating on a spell ... (PHB, p. 168)

From the above, it does not appear that "spellcaster" would necessarily be considered a class or group of classes, so

Are characters that can cast spells, regardless of their class(es), considered spellcasters?

Or, put another way,

Does "spellcaster" (as an attunement prerequisite) mean any creature who can cast a spell or does it mean all classes with the Spellcasting feature?

I can think of five particular cases—innate abilities, two subclasses and two feats:

High Elves know one wizard cantrip (0-level spell) of their choice. Drow can cast the dancing lights cantrip, 1st-level spell faerie fire and 2nd-level spell darkness. Forest Gnomes know the minor illusion cantrip, and Tieflings can cast the thaumaturgy cantrip and 2nd-level spells hellish rebuke and darkness. Also, many monsters have innate spellcasting abilities, which would trigger the Mage Slayer feat.

Starting at 3rd level, both the Eldritch Knight (Fighter Martial Archetype) and Arcane Trickster (Roguish Archetype) gain the ability to cast wizard spells and gain wizard spell slots (potentially) up to 4th-level spells. PHB, pp. 75 & 98. The latter's 17th-level Spell Thief feature also states "you gain the ability to magically steal the knowledge of how to cast a spell from another spellcaster," (Ibid.) which strongly implies that (at least) the Arcane Trickster is a spellcaster, although neither subclass is specifically listed as a prerequisite for any magic item in the DMG.

Eldritch Knight and Arcane Trickster are also found under Multiclassing, Spell Slots in the PHB:

You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. (p. 164)

With the Ritual Caster feat, a character chooses a spellcasting class (except paladin or ranger) and gains the ability to cast spells of that class with the ritual tag as rituals. The character immediately gets two 1st-level ritual spells of that class and has the potential to (eventually) learn all of the ritual spells of the chosen class. PHB, p. 169.

With the Magic Initiate feat, a character chooses a particular spellcasting class (except paladin or ranger) and immediately learns two cantrips and one 1st-level spell from the class with no further advancement. PHB, p. 168.

By definition, it seems all of the above would qualify as spellcasters, but I feel spellcaster typically (or maybe traditionally) refers to bard, cleric, druid, paladin, ranger, sorcerer, warlock and wizard.

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1. I believe an exception would be a 13th level or higher Thief (Roguish Archetype) due to the Use Magic Device feature. PHB, p. 97.

Answer 1

Straight from the DMG Errata:

Attunement (p. 136)

The first paragraph ends with a new sentence: “If the prerequisite is to be a spellcaster, a creature qualifies if it can cast at least one spell using its traits or features, not using a magic item or the like.”

emphasis added

Therefore, for the purpose of attunement:

If a creature is able to cast a spell by virtue of their own ability (not from an item), whether that be race, class, or feature, they are a spellcaster.

Answer 2

This answer was created to account for the changes made to the DMG through official errata published by Wizards of the Coast.

According to the newly errata'd DMG under attunement (p.136):

If the prerequisite is a class, a creature must be a member of that class to attune to the item. If the prerequisite is to be a spellcaster, a creature qualifies if it can cast at least one spell using its traits or features, not using a magic item or the like.

If the prerequisite is a specific class, it is pretty straightforward to determine who can attune to the item. Indeed, only creatures with levels in that specific class will be able to do so. Certain items, however, only require one to be a "spellcaster", without specifying anything. How then, does one qualify as such a spellcaster?

Qualifying through class features

For many players, qualifying as a spellcaster will be achieved through the "spellcasting" feature granted by many classes. Bards, Clerics, Druids, Sorcerers and Wizards all gain it at level 1; Paladins and Rangers gain it at level 2; Fighters and Rogues of appropriate paths gain it at level 3. Through this feature, such characters will gain access to cantrips and higher level spell, which they can either cast at will or by expanding spell slots. Similarly, Warlocks will gain the "pact magic" class feature at level 1, allowing them to cast spells through their Patron.

Some other classes, such as Barbarians and Monks, can gain the ability to cast certain specific spells (sometimes as rituals). While they often use different mechanics to cast these spells, such as Ki points for monks, the wording is usually very specific. For an example, one can look at the Way of the Four Elements monastic order (PHB, p.80) which specifically states that "some elemental disciplines allow you to cast spells", even referring the reader to chapter 10 for more details on spellcasting. One such discipline would be the Fist of Four Thunders, which allows you to cast the Thunderwave spell for the cost of 2 ki points.

This is to be contrasted with other features, such as the Way of Shadow monastic order's Cloak of Shadows, which allows you to become invisible without ever mentioning the casting of a spell.

Qualifying through feats

Another method of qualifying as a spellcaster is through the use of feats. According to the PHB, the following feats grant limited magical abilities :

• Magic Initiate: Grants 2 cantrips and 1 first level spell.
• Ritual Caster: Allows a character to learn and cast a certain number of spells, provided it is performed as a ritual.
• Spell Sniper: Grants one cantrip, with the caveat that one must basically already be a spellcaster to obtain this feat.

As the requirement clearly states that the spells must be granted through features or traits, using these feats would indeed allow a character to qualify as a spellcaster. This is because feats are optional rules that can be used either as a substitute for the Ability Score Improvement class feature of every class or as an alternate racial trait for humans.

Qualifying through racial traits

Finally, certain races are innately able to cast a certain number of spells. Such races include, but are not restricted to: High Elves (1 cantrip from the wizard spell list), Drow Elves (dancing lights cantrip, Fairie Fire and Darkness), Forest Gnomes (minor illusion cantrip) and Tieflings (thaumaturgy cantrip, hellish rebuke, darkness). Being able to cast these spells through the use of a racial traits qualifies such races as spellcasters according to the errata.

What about monsters?

In the Monster Manual, some monsters, such as the Deva (MM, p.16), possess the "Innate Spellcasting" trait, which grants them the ability to cast certain specific spells under certain conditions such as limited uses per day, restricted to self-only, etc. Since these monsters are able to cast spells and that they are granted through a special trait, they would indeed qualify as generic spellcasters for the purpose of attunement.

Other monsters, such as the Death Knight (MM, p.47), possess the "Spellcasting" trait, which usually specifies both a class and a level (in the Death Knight's case, these are "Paladin" and "19th-level"). As such, these monsters not only qualify as generic spellcasters, but also as a members of these specific classes for the purpose of attunement. For example, a Death Knight could attune himself to paladin-only items.

What does the part about "[...] not using a magic item or the like" mean?

In the DMG, magical items such as the Luck Blade (p.179) can be used to cast certain spells without themselves requiring attunement by a spellcaster or a member of a specific spellcasting class (it only requires "ordinary" attunement). This condition has thus been added to the rules so that creatures attuned to these items would not be considered generic spellcasters. In a sense, it is implied that it is the item that casts the spells upon demand, and not the creature carrying the item.

Answer 3

The DMG errata published in December 2015 invalidates many elements of this answer. For more up-to-date references, please consult New_DM_Tryingtobesneaky's answer, or my own alternate answer.

TL;DR

I believe that there are enough clues throughout the core books to indicate that "spellcaster" refers to "member of a spellcasting class", which can be extrapolated to mean "character with the spellcasting class feature". This allows to differentiate proper spellcasting from racial or monstrous magical abilities (which can have many different sources), as well as spells cast as the result of awesome wushu techniques (using ki points, to boot!). If some monsters were meant to be considered spellcasters, their descriptions would include spell slots like other spellcasters, and not something along the lines of "can cast [insert spell name] once per day".

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While it would seem that, rules as written, any character or creature with the ability to cast spells could be considered a "spellcaster", there are many clues lying around the core books that indicate that, "rules as intended", it's probably not the case.

"Spellcasting" is a specific class feature.

Some classes gain the ability to cast spells through the "Spellcasting" feature. Bards, Clerics, Druids, Sorcerers, Warlocks and Wizards all gain it at level 1; Paladins and Rangers gain it at level 2; Fighters and Rogues of appropriate paths gain it at level 3.

Some other classes, such as Barbarians and Monks, can gain the ability to cast certain specific spells (sometimes as rituals). While they do gain the ability to wield magic, sometimes even using different mechanics, they never acquire the "spellcasting" class feature. While the rules do not explicitly say so, these classes are not traditionally considered spellcasters and it makes sense that they shouldn't receive the "spellcasting" feature.

Races with magical abilities are not described as "spellcasters".

In the Player's Handbook, some races possess what could be described as innate magic. Common examples would be high and dark elves, as well as forest gnomes. While all of these races gain the ability to cast a certain selection of spells, they do so through a racial ability called "cantrip" for high elves, "drow magic" for dark elves or "natural illusionist" for gnomes.

While this argument in itself cannot completely invalidate the "spellcaster = able to cast spell" hypothesis, one needs to remember that 5th edition is usually very thorough where language is concerned. If these races were meant to cast spells in the same way the spellcasting classes did, they would gain their spells through a racial feature called "spellcasting". As it is currently written, I believe it is strongly implied that these races have their own magical ways that are entirely different than those of traditional spellcasters.

Indeed, elves are able to cast these spells whether or not they study magic through tomes, nor do they require specific bloodlines, pacts or religious/natural worship to do so. Their abilities are part of them, and mechanically replicate spells as it's much simpler than creating completely new magics.

The Dungeon Master's Guide often refers to spellcasters as possessing spell slots.

In the "Crafting a magic item" section, it is stated that "the character must be a spellcaster with spell slots and must be able to cast any spells that the item can produce." Note that, technically, a drow with some wizard levels (or even a multiclass monk/wizard) should be able to cast an item that grants the darkness spell even though he has never learned that spell as a wizard. This is because his racial feature specifically mentions that "[he] can also cast the darkness spell once per day".

Reading the Manual of Golems requires a "spellcaster with at least two 5th-level spell slots". Failure to do so results in massive damage for the would-be reader. The Manual of Golems is a rare and very powerful item, and its use is thus restricted to only the most powerful of spellcasters. With the description provided, it would appear that the strength of a spellcaster is measured in the quantity and level of the spell slots he can use to cast spells, something that is only granted through the "spellcasting" class feature.

The Rod of Absorption also specifies that a spellcaster can use it to "convert energy stored in it into spell slots to cast spells you have prepared or know". If magical races were considered spellcasters, this item would still be pointless for them. Indeed, using a spell slot to cast a spell requires the "spellcasting" feature. As such, even if a drow would use the Rod of Absorption to generate spell slots, he still wouldn't be able to use them. Note that this item does not allow someone to replenish his uses of racial magic.

While not all magical items requiring attunement by a spellcaster are so explicit in their description, there are enough passages that are to heavily imply that, rules as intended, spellcasters need to possess spell slots, and thus the "spellcasting" class feature.

The attunement rules are very specific as to what creature can attune to magical items.

In the attunement section of the DMG, it is stated that a character might have to satisfy some requirements before being able to bind some magical items to himself. It is also stated that "if the prerequisite is a class, a creature must be a member of that class", with the provision that "if the class is a spellcasting class, a monster qualifies if that monsters has spell slots and uses that class's spell list".

Some items, however, do not mention a spellcasting class specifically as a prerequisite. Instead, they refer to "spellcasters". While this may cause some confusion, I believe, given the ample clues to that effect throughout both the PHB and DMG, that a "spellcaster" is intended to be a catch-all term to refer to "a member of a spellcasting class". Indeed, if you wanted to restrict some magical items to the classes that can wield magic, naming all of them, including the specific cases of classes gaining the spellcasting ability at later levels or through specific career paths, would rapidly become very tedious and clutter the book tremendously. "Spellcaster" is short, and basically says what it means if you consider the global context of 5th edition.

Monsters, using this definition of "spellcaster", can thus only attune to these items if they possess spell slots, and then only if their description indicates that their spells are from any spellcasting class's spell list, as no class is specifically mentioned.

Answer 4

If the ability to cast a spell comes from your class, then you meet the prerequisite for attunement's purposes.

Since attunement's prerequisite is based on class, the ability to cast one or more spells granted by something other than a class doesn't satisfy the class requirement. (The exception for monsters can be ignored for PCs.)

So Eldritch Knights and Arcane Tricksters, being able to cast spells due to a feature that comes from their class, qualify as class-based spellcasters and can attune magic items that require a spellcasting class.

(Non–Eldritch Knight fighters' and non–Arcane Trickster thieves' classes don't qualify them as spellcasters, as they can't cast spells even if the class has the potential to grant them spellcasting.)