It is based on an imitation game in which a man (A) a woman (B) and an examinerA.1 (C) communicate via (typewritten) text messages. The objective of the examiner is to decide which is which by constructing smart questions. A computer is supposed to take the place of the man (A). The objective of (A) and (B) is to convince the examiner that it/she is the human person. According to the proposal the computer can be ascribed intelligence when it succeeds in convincing the examiner that it is the human.
Because of some objections as to the relevance of this question, you often find the additional requirements that: First, (C) should be given adequate time. Also (B) should be well capable of expressing herself, since someone can be intelligent without being able to express this well enough over a `chat session'.
Erik de Bruijn