A wall that is always being broken so reality meets fantasy. Like in Anime and Cartoons.
Anime like Naruto, Pokémon, Bobobo-bo Bo-bobo and Duel Masters break the fourth wall.
Cartoons Ed, Edd n Eddy, Powerpuff Girls, Brandy and Mr. Whiskers and Yin Yang Yo break the fourth wall.
They bring up the word "episode" sometimes, too.
They look at YOU directly and they mention their own show!
Every Anime and Cartoon has a fourth wall... It's just that the directors and producers don't have them break it. It's not in their script.
The thin line that exists between a story and reality. When a character in a story tells the reader in some way that they know that they are a character in a story, that is called 'breaking the fourth wall'.
Character 1: The author is a jackass who can't write me properly.
Character 2: Well, there goes the fourth wall.
In theatre, the side or sides of the set facing the audience that have no physical reality. The audience and actors aknowledge this deliniation as a convention of theatre, unless the director chooses to "break" the fourth wall.
An actor making eye contact with the audience is breaking the fourth wall.