Can characters/NPCs understand what a crit (Nat 20) is? If so, can they tell that Adamantine Armor negates crits?
Context: I'm trying to figure out if there's any good reason Tier 4 PCs wouldn't bring Adamantine Armor with them when using Astral Projection.
There is no real backup to this, critical hits are entirely a game term. There is no evidence behind characters knowing game terms, because that would be metagaming (as referenced in other similar questions, already linked in the comments).
However, it's undeniable that some hits do in fact hit harder than others. In my experience, this is usually explained by a skillful hit, like finding a small chink in someone's armor and managing to get a good stab in - a difficult feat that requires a bit of luck. Another example would be getting everything right: posture, form, breath out when swinging down, etc. Something that can be hard to accomplish in the heat of battle when someone else is throwing fire at you, but can certainly give you the upper hand.
As far as explaining how Adamantine Armor prevents that, it's magic. There are no flaws in the armor to exploit, thanks to magic. And, because of this magic, the protection extends to your already neigh-uncut-able silver cord even further, preventing just about anything from cutting it -- even a Githyanki Knight's silver longsword, which required the knight to get lucky and perform some maneuvers to get behind you to even get the chance to slice it (to get a critical hit).
A character absolutely knows they scored a critical hit.[1]
When I was an undergraduate, I fenced competitively (primarily foil, but I sometimes dabbled with epee). In foil, there are a lot of rules which determine whether or not a "hit" actually counts as a touch:
If you manage to hit your opponent on valid target, with right of way, and so on, you are awarded a "touch" (a point, if you will); a bout goes to 15 touches. Often, you will score valid touches on the very edge of the valid target, or you'll hit after a sloppy parry, or you'll hit your opponent just before they hit you. I once tripped over an untied shoelace, launching myself forward, but managed to keep my balance just long enough to hit someone on their side before falling over. Fencing can get incredibly messy but, as long as you follow the rules, you get the touch. In D&D terms, you beat your opponent's AC and did damage.
Sometimes, however, your technique, strategy, and execution all come together, and you score a really nice touch. Maybe your opponent attempts to parry, but you slip your blade just under theirs and hit them right in the middle of the chest. Or you launch an aggressive fleche and nail your opponent on back with a well-timed flick as you run past them. Perhaps you make a really nice feint, provoke an attack, parry in seven, and nail your opponent in the belly button. At a North American Cup in 2003ish, I fenced a bout against Sean McClain—he was maybe just past his prime, but still one of the best fencers in the world, and I was never going to win that bout. But I did manage to score a single, solitary touch—it was clean, well executed, and felt amazing. These kinds of touches feel good. You know, in your gut, that you dominated your opponent (if only for a moment).
That's the feeling of a critical hit.
[1] There seem to be a couple of questions being asked here. I am going to focus on the question in the title, which is about whether or not a character knows if they have scored a critical hit.
A critical hit, or armor class, hit points, saving throws, ability checks, etc. are the manifestation in our world of something happening in the game world. The characters don’t understand these in the exact same way as the players, but they do understand them.
The mechanics are necessary for differentiating an RPG from simple make-believe play. What does a goal represent in football and why should the team that has more of them win after 90 minutes. Who cares! Both are just games and those just happen to be the rules.
does a character know getting stabbed in the eye hurts more than in the foot, I can't think of a reason why not.
Remember game terms may not be meaningful the concepts behind them should be. Do they know a getting stabbed in the eye is a "critical", likely not, but they know it hurts more and adamantine protects against it.
I don't want to rip off a post whole hog, so go read this answer about spell slots and the murky mirror. https://rpg.stackexchange.com/a/131734/37168
Your accepted answer is fine, but let me try a different angle:
The whole fighting system of DnD is perfectly out-of-character. It is a very rough approximation of what actual combat would look like. It abstracts away basically all details and hides them behind probabilities (i.e. D20) and difficulty measurements (i.e., AC) and very crude measures for how good or bad a character is at something (i.e., stat bonuses, proficiency bonuses).
In this regard, DnD 5e is wildly different (simpler) than other systems; for example ones that track damage to individual body parts.
If you think about it, AC is ludicrously crude of a measure, at face value. You are telling me that if someone hits my character there is nothing in the world he can do except hope for the attacker missing?
But all of the previously said is of course a mis-interpretation. AC in truth contains all imaginable ways an attack (and the defense thereof) could play out, from the attacker just being a much better fighter, over me having much better armour, or being faster, or very small or large, or whether a character carries a shield or not, or has scales, and so on. DnD 5e just does not want to spend so much time and complexity in the rules about this and lumps all of it together, and thus opens up plenty of opportunities for role-playing (describing) what happens in-game.
And Critical Hits are just the same: they encapsulate everything that could go so beautifully right for the attacker and could mean that they succeed against otherwise impossible odds. They abstract happy chances away, they hide "pure luck", they contain epic unforeseen story-telling events and so on and forth, all behind a single mechanism.
So yes. In our group, our characters definitely know when they performed a critical hit; the GM will definitely ask us to describe the attack (or does a colorful description themselves) while the player gleefully rolls their bonus damage and enjoys the spotlight.
The characters need no concept of this. They already have the concept that sometimes unexpected great things happen; that the worst person can have the occasional uber-lucky strike, that the awesomest Fighter can occasionally still be hit by a peon due to absolutely bad luck, and so on and forth. They only count on it if they are very foolish (just like the regular buyer of lottery tickets in Real_Life[tm]).
If they have deep insight about that Adamantine Armour, they can very well know (if this is part of what is known in the world at all) that nobody ever, EVER had any luck hitting the wearer of this armour, not even the gods. Or they can learn by experience that over and over again, even if a monster hit them in the most severe fashion whatsoever, the Adamantine still was able to deflect a blow that would have ended any defender wearing a lesser piece. Or they can know some blacksmith who foolishly tried to damage a piece of Adamantine for science, and only managed to break his smithing hammer in two.
As to what keeps your party from bringing that stuff everywhere by default, I guess that's up to the DM, in general - if they want them to not do it, they surely can come up with some direct or indirect cost that makes it unfeasable sometimes.
By request, some sources (in the DnD 5e Player's Handbook):
You say:
Probably not.
TLDR: Given a chance, those PCs for whom armor is appropriate would likely be fully kitted in it, or maybe carrying a set just in case. They might or might not have every bit of information regarding adamantine armor and its advantages and disadvantages compared to other choices, but they likely have every bit of information that they had the time and resources to gather. So, it's really a question of PC sleuthing, DMing, and world-building.
Tier 4 PCs (levels 17-20) are described as "masters of the world":
By 17th level, characters have superheroic capabilities, and their deeds and adventures are the stuff of legend. Ordinary people can hardly dream of such heights of power — or such terrible dangers.
In my experience, such characters will do everything in their power to face a significant danger with every tool that can be obtained in the given time and with the given resources.
Adamantine armor is merely uncommon. With the defaults presented in the DMG and other source books, Tier 4 PCs, masters of the world, would likely have broad knowledge of uncommon magic items, and so would most likely know of adamantine armor.
Characters with reason to do so, such as those who could wear such armor, given time and reason, would likely know details about it. Its properties. How it compares and contrasts to other expensive armor. Where to obtain.
You need to decide what level of knowledge they have, and is it automatic, a knowledge check, a conversation with an NPC, or a side quest, or potentially, just not available.
Average inhabitants of the default setting might or might not know much about the Astral Plane. Likely their knowledge is rudimentary, apocryphal, fragmentary, and incorrect.
However, Tier 4 PCs are masters of the world, and by definition "traverse otherworldly realms and explore demiplanes and other extraplanar locales". They are likely to have broad knowledge of the planes of existence.
Characters with reason, such as those who could travel to other planes, or who have such connections, or who previously studied such things, would likely know details about the Astral Plane. It's properties. It's inhabitants and dangers.
You need to decide what level of knowledge they have, and is it automatic, a knowledge check, a conversation with an NPC, or a side quest, or potentially, just not available.
So, Tier 4 PCs are quite likely to have general knowledge of adamantine armor and the Astral Plane. How extensive that knowledge is, is up to you. And up to the PCs, for that matter. Given time and resources, they can likely find out most detail available. They are likely to then equip themselves as best as possible.
Do they know of natural 20s? Do they conceive of themselves as characters being played by creatures of a higher reality? Whew, you could play it that way. It is a stable of fiction that we mortals are merely pawns of a bigger game played amongst the beings of a higher order of existence.
The rules are quite muddled on the difference between player and character. For instance, the rules routinely change perspective, sometimes talking about players, sometimes talking about characters. From the introduction to the Basic Rules, "How to Play":
Sometimes one player speaks for the whole party, saying, “We’ll take the east door,” for example.
Really . . . it's the character talking about the east door. Or is it?
The player knows a 20 on a die is a critical. The character knows some attacks are better than others.
And any character who's had a chance to prepare, will know that going into the Astral Plane involves significant risk to that precious silver cord, and may will choose to try to mitigate that risk though any way possible, including guarding against the skillful or lucky hits that might sever the silver cord.
I know I would. I mean, my character would.
