A tautology is a statement that says the same thing twice using different words, so the second part adds nothing: 'he sat alone by himself', 'a new innovation', 'at 8am in the morning'. The word is countable — you can find three tautologies in one paragraph — and it is also used uncountably for the habit itself ('the report suffers from tautology'). It is a formal word; you meet it from editors, language teachers, and style guides. In everyday speech people just say 'that's repeating yourself'. In logic it has a tighter meaning: a statement that is true no matter what, because of its own structure — 'she will either come or she won't'. Logicians do not treat that as a fault; it is simply a statement carrying no information about the world. Worth knowing that native speakers use tautologies constantly in speech for emphasis or rhythm, and nobody minds. It is mainly in careful writing that they get cut.

同義反覆、贅述（用不同的字把同一件事講兩次；邏輯學上指「恆真句」）