fourth wall
The imaginary "wall" that separates a piece of fiction from the real-life audience.
This tag is used when that wall is "broken" by the characters involved indicating they realize that they are set in fiction, usually by addressing the audience or interacting with elements of the medium, such as grabbing a speech-bubble or similar actions.
Deadpool in particular is known for his constant breaking of the fourth wall.
The pixiv tag is メタ発言 (meta quote). However, "meta" tag on danbooru has a bit different meaning.