'一定' (yídìng) is an adverb meaning 'certainly', 'definitely', or 'for sure'. Literally, '一' means 'one' and '定' means 'to fix' or 'to decide', so together they convey the idea of something being firmly settled or unchangeable — hence certainty. It’s commonly used to express strong confidence in a statement, a promise, or an intention, often adding emphasis and sincerity.

This word frequently appears in spoken and written Chinese to strengthen assertions, make commitments ('I will definitely do it'), or reassure others ('It will certainly be fine'). It can also function as a noun meaning 'a certainty' (e.g., '这是一定的' — 'This is certain'), though this usage is less common for learners. Unlike English 'definitely', '一定' usually precedes the verb or adjective it modifies and cannot stand alone as a response without context.