Unless is a conjunction used to introduce a condition that must be met for the main clause to be true or valid — or, more commonly, to indicate that the main clause will happen only if the stated condition does *not* occur. Literally, 除 means 'to remove' or 'except', and 非 means 'not' or 'false'; together, 除非 conveys the idea of 'except if not', which evolved into the modern meaning 'unless'. It always introduces a subordinate clause and is followed by the main clause, often with a contrasting or conditional relationship.

This word is common in both spoken and written Chinese, especially in formal explanations, warnings, or logical reasoning. Unlike English 'unless', 除非 cannot stand alone at the beginning of a sentence without a following clause — it requires a full conditional structure (unless X, then Y). It’s frequently paired with 就, 才, or 也 to strengthen emphasis or clarify consequence.