Ludonarrative Dissonance

Professor Oak not allowing the protagonist to ride a bicycle indoors even though they are capable of doing so anywhere else.

Game Mechanics is a term used in reference to games (typically video games) where there are abilities determinant on the ruleset of a game (i.e. hit points/health points, levels, stats, instant death to bottomless pits, save files, item or technique limitations, etc.) that aren't viable for usage as part of the feats of a character.

For example, in the Golden Sun series of games, you are able to equip different Djinni to give you access to different classes and techniques that you can utilize. The Hermit class has access to moves like Hail Prism while the Ninja case doesn't. This is because of the limitations imposed by the game to only allow you to equip a certain amount of Djinn to make fights more strategic for the players. Outside of the game, they would be fully capable of equipping all of the Djinn to access all of the different classes and moves since it's just a game mechanic.

Ideas such as health points are largely game mechanics as mentioned earlier as well. This is because of the idea of being able to wither down somebody isn't realistic to how durability functions in reality. In most cases, the idea of withering down somebody would just imply that you are comparable to them, not that everybody else who would attack that person would be able to do the same thing. If you whittle down a Galaxy level being's health, you'd be comparable in that you are also likely Galaxy level. However, if a Planet level character tried the same thing, it would fail, and they'd ultimately not be able to harm them.

Game Mechanics are limitations and features imposed for the game to operate, using them in any sort of debate or profile is fallacious and inaccurately representing a character.

It should be noted, however, there are cases where what might be a game mechanic might otherwise be treated canon by a game. Undertale is a good example of this as while save files are usually just relegated as being game mechanics, the game makes numerous references to them as characters acknowledge the mechanic and use it themselves. OFF is another example where there are open references from characters knowing of the presence of characters such as the player acknowledging these mechanics as something present. Mechanics can be canon, but they require substantial proof in order to be considered.

For a more in-depth explanation on the subject, here is an article defining what Game Mechanics are.