Liar's paradox

Liar's paradox is the paradox discovered by the Greek Stoics of which sentence (i) is the simplest example:

(i) Sentence (i) is false

For sentence (i) to be true, sentence (i) has to be false. Conversely, if sentence (i) is false, it has to be true. The liar's paradox can be avoided by prohibiting that an expression refers to itself, i.e. by making a strict separation between object language (the language as object) and meta language (the language as medium).

Link
Utrecht Lexicon of Linguistics