Generally speaking any interactive door the player can open and close. Some games may be limited to only opening doors. Others may have automated doors, and so forth.

Gates are not exactly the same thing, but function similarly and serve pretty much the same purpose, which is why they're included in this.

NOT closet doors or such.
NOT for doors that cause level change (this may be ignored in older games, but a note should be added about it).
NOT for unusable doors in background (even if they make a noise when you attempt to open them)
NOT for doors only usable by NPCs (except maybe in strategy games).
NOT for doors that only open but never close (common in older games), these are indistinguishable from walls being blown to bits or fences knocked down.

Related:
* Keys
* Keycodes