Tautological describes a statement that says the same thing twice in different words, when once was enough — 'he sat alone by himself', 'a free gift', 'the reason is because'. The second half adds no new information; it just repeats the first. Calling something tautological is a mild criticism: you are saying the writing is padded, not wrong. It is a formal word, used by editors, teachers, and writers about language. In ordinary speech people say 'you're repeating yourself' or 'that's saying the same thing twice'. There is also a stricter use in logic and philosophy, where a tautological statement is one that must be true because of its own shape — 'either it will rain tomorrow or it won't' — true whatever happens, and therefore telling you nothing. Both uses share the same core: words that go in a circle. You will also see 'tautologous', which means the same; tautological is the commoner form.

同義反覆的、贅述的（把同一件事用不同的字說兩遍，等於白說）