In it’s strictest sense, a tautology is a statement which is empty of meaning because it is self-evidently true. The kind of example which is presented in philosophy is A woman is female
and Venus is the evening star.
However, in general use, the term has extended to include identity of meaning between sentences, and taken on a pejorative sense, to mean unnecessary
repetition. This entry ought therefore to be read in conjunction with the one on circumlocution
Not all tautologies are to be condemned. There are times when I myself personally would sound all right - such as in a jocular response by a husband to his wife’s surprised question ‘Who did the washing up?’ And all definitions are tautologies, such as Linguistics is the science of language. Alas, some tautologies are only apparent: a deeper meaning is being signalled by the utterance. Boys will be boys is saying more than it seems. So is Once I’ve promised, I’ve promised and When I go out drinking, I go out drinking and My sister is a Lady. And if someone says I don’t like that proposal, and follows it up with That proposal I definitely don’t like, there is an extra element of definiteness and emphasis which would be lacking if only one of the two utterances had been made.
In literature, tautology can be used to convey something about character and attitude. A famous example is Hamlet’s comment: There’s ne’er a villain dwelling in all Denmark / But he’s an arrant knave.