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I'm going to use one example just to demonstrate the relevancy of this question, but I want to make clear that this one example does not constitute the complete domain of my question; nor do the other questions I linked in this post constitute the whole picture of what I'm asking, but instead are just illustrative examples.

The Inciting Example

For a creature like a Panther, their Dexterity Bonus is +2, and their CR is 1/4, meaning they'd normally have a Proficiency Bonus of +2, meaning Dexterity-based skills that they have Proficiency in should have a bonus of +4. However, for Stealth, they have a total bonus of +6.

Mechanically, this is justified: the Monster Manual expressly calls out monster skills as potentially subject to greater-than-expected bonuses, borne out of expertise in the given skill:

A skill bonus is the sum of a monster's relevant ability modifier and its proficiency bonus, which is determined by the monster's challenge rating (as shown in the Proficiency Bonus by Challenge Rating table). Other modifiers might apply. For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus) to account for its heightened expertise.

Skills, Monster Manual, pg. 8

So we have a clear explanation for why a Panther would have an unusually high Dexterity (Stealth) bonus of +6, but this also leads to some strange territory in terms of game mechanics. For example, if a Panther is tamed by a Beast Conclave Ranger (as seen in Unearthed Arcana: Revised Ranger), the Panther would substitute their master's Proficiency Bonus for their own for their skills. This leads to an ambiguous situation regarding what bonus the Panther should have to this skill:

  • 4, because you use the Ranger's Proficiency bonus, replacing the Panther's getting 2(DEX) + 2(PROF)
  • 6, because you use the Ranger's Proficiency bonus, and then double it, because the Panther has expertise in Stealth, getting 2(DEX) + 2x2(PROF)

And then at level 5, this situation arises again, does the Panther have

  • 5, because you use the Ranger's Proficiency bonus, replacing the Panther's getting 2(DEX) + 3(PROF)
  • 8, because you use the Ranger's Proficiency bonus, and then double it, because the Panther has expertise in Stealth, getting 2(DEX) + 2x3(PROF)

And so on at higher levels, does this ambiguity continue to arise.

For Completeness

This is a relevant question for lots of other classes as well. Druids, for example, are able to Wild-Shape into various beasts; a Druid could shape-shift into a Panther, and then the Druid's Proficiency Bonus (which at most levels of play will be higher than the Panther's proficiency) might or might not get doubled: see this question for a concrete example: When a druid wild shapes to a beast that has double proficiency with a skill, should the druid's proficiency bonus be doubled with that skill?

'Expertise' vs 'expertise'

In a recent answer I gave about Aboleths having a doubled proficiency bonus in History, I very explicitly avoided saying that Aboleths "have expertise" in those given skills, because I was worried that the answer might get downvotes over arguments about the semantics of a creature "having expertise" or not, and instead I focused on describing them as "having something that is mechanically equivalent to expertise" in those skills.

So what I'm most curious about is the use of that original passage I quoted above. It connects a creature potentially doubling its Proficiency bonus to the creature having 'expertise' in a given skill, but it's not made clear that 'expertise' represents a tangible feature of the creature in question. Ordinarily, I would be inclined to argue that the term is meant to just be a colloquialism, but then we know that Bards and Rogues have an explicitly named feature, Expertise, that explicitly allows these classes to double their proficiency bonus on specific checks.

Then there's a feat featured in Xanathar's Guide to Everything, Prodigy, which strongly implies that 'expertise' is a concept, not merely a colloquialism:

Choose one skill in which you have proficiency. You gain expertise with that skill, which means your proficiency bonus is doubled for any ability check you make with it. The skill you choose must be one that isn't already benefiting from a feature, such as Expertise, that doubles your proficiency bonus.

Skilled, Xanathar's Guide to Everything, pg. 75

So it seems very strongly like the word 'expertise' has a Formal Ontology in 5th Edition D&D, meaning "double your proficiency bonus for checks made using this skill", and is not merely a colloquialism for "your bonus in this skill is higher than expected".

But then I look at answers, like in the Druid question I linked, that seem to imply that this is not the case; that it really is a colloquialism, and that there is no formal concept of a feature that doubles a creature's proficiency bonus, except in the very specific case of Rogues and Bards with their explicitly named feature, Expertise, providing such a benefit. And of course, my Aboleth answer above focused on the same kind of rhetoric, without receiving pushback on the semantics.

So which is true? Does the word 'expertise' have a formal ontology in 5th Edition, or doesn't it?

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    \$\begingroup\$ This question is extremely long winded for essentially asking the very simple question of if "expertise" is a defined game term throughout. I really think you can and should cut the text back substantially. \$\endgroup\$ – Rubiksmoose May 17 at 22:36
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    \$\begingroup\$ In addition to Aboleth, there are Knowledge Clerics, Scouts, and Purple Dragon Knights who get "double their proficiency bonus" in skills but the word "expertise" is not used in any form. \$\endgroup\$ – krb May 18 at 3:08
  • \$\begingroup\$ I wish I could award a bounty to this question as it challenges the frame of so many things and is substantially more than a long winded way for asking a simple question. \$\endgroup\$ – Akixkisu May 29 at 11:57
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D&D doesn't have "Formal Ontology".

This is a book about how to play make-believe, not a geometry textbook. None of the terminology has rigorous definitions.

If "expertise" has a "Formal Ontology" then it should be defined somewhere, and it's not.

The mechanic commonly referred to as "expertise" was introduced like this:

Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. For example, the rogue’s Expertise feature doubles the proficiency bonus for certain ability checks. If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you still add it only once and multiply or divide it only once.

This would have been exactly the right place to say "If you have 'Expertise' then that means you double your proficiency bonus", but they instead referenced the class feature Expertise as an example of something that would give you double proficiency. This is pretty strong evidence that "Expertise" was originally meant only as the name of the class feature.

Now, we've found here (as in the question you linked about the Aboleth) that it's a useful shorthand, especially since the Expertise class feature is just about the only way to get double proficiency in anything. Whoever wrote that section in Xanathar's apparently found the same thing and adapted their usage accordingly. Now we could say, since there's a published source with the D&D trademarked logo and everything, that "expertise" means "double proficiency" (while noticing that it's still also the name of a class feature, and it sounds really silly to say "the Expertise class feature gives you expertise"). But there's a big difference between a commonly accepted shorthand and a Formal Ontology.

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    \$\begingroup\$ I feel like this answer relies on a strange priority of authorship. "Whoever wrote that section in Xanathar's" also had their work vetted by editors and proofreaders as did the Player's Handbook; why is the use in Xanathar's, where it does appear to be used as a formal concept, presumed to be at the whim of an arbitrary copyeditor rather than as a demarcation of intent like with other rules found in these books? \$\endgroup\$ – Xirema May 17 at 21:27
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    \$\begingroup\$ I'm not saying the Xanathar's usage is arbitrary, whimsical, or badly edited in any way the PHB isn't. I'm saying that the use of language in all of these books is informal enough that the PHB can use "expertise" to mean one thing, Xanathar's can use it to mean something related but clearly different, and neither of those meanings is "correct", because this isn't a formal system. \$\endgroup\$ – Mark Wells May 17 at 22:18
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    \$\begingroup\$ It's also worth keeping in mind that in Xanathar's, the two places it seems to use "expertise" as if it were a game term also immediately also define it as doubling the proficiency bonus. I agree that it's being used as a "useful shorthand", and that it's not a general game term in the way that "proficiency bonus" is. \$\endgroup\$ – V2Blast May 18 at 7:39
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It does not.

Jeremy Crawford has addressed this a few times in different places, and the general idea he's always pushed is that 5e doesn't have very many specific game terms. Words generally just mean what the word normally means unless it's one of those few game terms, and specific game terms are defined in places like the Combat chapter of the PHB or the appendices, not hidden in some corner of a class.

The Rogue has a specific ability called "Expertise", but outside the context of that specific class, there is no such thing as "expertise" as distinct from the English meaning of the word. In other words, there is no game-wide concept of "Expertise" in the "double your proficiency bonus" sense.

Monsters don't have "Expertise"; they just sometimes have a "larger-than-expected bonus" as compared to their CR-based proficiency.

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    \$\begingroup\$ How do you reconcile this claim against the Prodigy feat I quoted, which specifically says "you gain expertise in this skill, which means your proficiency bonus is doubled"? \$\endgroup\$ – Xirema May 17 at 21:04
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    \$\begingroup\$ @xir I think it's relevant to point out that the Prodigy feat uses the term "expertise" and then goes on to define it. It does not rely on an outside pre-existing mechanic/definition of expertise. \$\endgroup\$ – Rykara May 17 at 21:07
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    \$\begingroup\$ @Rykara That's fine; but tautologically, would the term not become a formal term, as a consequence of the Prodigy feat choosing to define it? \$\endgroup\$ – Xirema May 17 at 21:13
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    \$\begingroup\$ @Xirema Prodigy uses the english meaning of the word, and then explains what it does in rules terms. There is no conflict in need of reconciliation. Things don't become formal terms just because they use them in relation to specific rules a lot. \$\endgroup\$ – Darth Pseudonym May 17 at 21:20
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    \$\begingroup\$ A good example of a word having "Ontology" is "move". In which it isn't even consitent whether this means the game defined "expends movement" or the English Dictionary definition. See this question, its approved answer and the first comment on said answer: rpg.stackexchange.com/questions/122943 \$\endgroup\$ – Medix2 May 18 at 15:43
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Expertise is not a defined game term outside of the class feature

The rogue class feature Expertise does exactly what it says it does and does not lay out any kind of general definition for the term "expertise" that could be read as applying to every other instance of the term in the book. Nowhere is the term "expertise" defined to mean anything special in general.

This falls in line with the general way 5e deals with game terms in that there aren't that many of them and that when you see a word that is not a clearly defined game term you are intended to interpret it according to the natural English definition of the word.

Examples of "expertise" not being used to talk about doubled prof bonus

If "expertise" were a defined game term meaning that proficiency is doubled, then you would expect that term to be used precisely and consistently throughout the rules. However, you can see many examples of "expertise" being used that explicitly and clearly have nothing to do with skill or proficiencies.

For example, this tidbit about gnomes:

However, their humor and enthusiasm are dampened by their oppressive environment, and their inventive expertise is directed mostly toward stonework. (PHB 36)

Clearly here expertise is being used as a general word for "an area one is really good at" rather than any given game meaning.

Similarly, the Entertainer background says:

Choose one to three routines or roll on the table below to define your expertise as an entertainer.

This has nothing to do with the game term "skills" or proficiencies, but instead is defining a background aspect of the character with little mechanical impact.

There are even other class features containing the word, like the Rogue Assassin class feature Infiltration Expertise which only has to do with your ability to disguise oneself and nothing to do with skills or proficiencies.

If you go and say that "expertise" has a formal definition and ontology in 5e it is going to result in the interpretations of these examples changing in significant and absurd ways.

Examples of doubled prof bonus not using the term "expertise"

On the flip side, if "expertise" was a defined game term you'd expect the rules to use it consistently to places where proficiency bonuses are doubled. This also seems to not be the case.

There are several places where creatures get double proficiency where "expertise" isn't even mentioned. For example the Rogue Scout's Survivalist feature doubles the proficiency bonus for nature and survival ability checks but never mentions "expertise". (thanks @alk) Knowledge clerics and Purple Dragon Knights have a similar feature as well that also doesn't mention "expertise".

Conclusion

Given that there is no formal or implied definition for "expertise" anywhere in the rules and the fact that it is clearly used as the common English definition in many contexts, we can conclude that it has no special game meaning outside of those areas where it is specifically prescribed that meaning.

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    \$\begingroup\$ Additional example: the spell Skill Empowerment from XGTE: dndbeyond.com/spells/skill-empowerment It both talks about doubling your proficiency bonus w/out calling it Expertise...then calls out Expertise as an alternative way you could have doubled Proficiency. \$\endgroup\$ – guildsbounty May 20 at 16:59
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    \$\begingroup\$ @guildsbounty actually, that spell does mention "expertise" in the first paragraph and contrasts that with "Expertise" in the last paragraph. That's an interesting example. \$\endgroup\$ – Rubiksmoose May 20 at 17:01
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    \$\begingroup\$ @Rubiksmoose Ah, right, missed that. Perhaps still worth noting as it seems to be treating expertise and Expertise as two different things. I.e. it doesn't say "And they must not be gaining Expertise from another source." \$\endgroup\$ – guildsbounty May 20 at 17:02
  • \$\begingroup\$ @guildsbounty for sure! I'll have to think how to fit it in. \$\endgroup\$ – Rubiksmoose May 20 at 17:02

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