Or in the original French, "Ceci n'est pas un pipe."

A piece of art by Rene Magritte, which has a picture of a nice tobacco-smoking pipe, and the title, "This is not a pipe" below it.

A very literalist notion; it is not a pipe, but a picture of a pipe. Visual representation has become blurred and indistinct from reality.

Also a book by Michel Foucault, which explores the art and this phenomenon in greater depth.