必定 is an adverb meaning 'certainly' or 'surely', expressing strong confidence in the truth or inevitability of something. The character 必 (bì) means 'must' or 'inevitable', while 定 (dìng) means 'to decide', 'to fix', or 'to settle'. Together, they form a compound emphasizing unwavering certainty — not just probability, but logical or factual necessity. It’s commonly used in formal speech, written Chinese, and reasoned predictions (e.g., based on evidence or universal principles).

This word often appears before verbs or adjectives to modify them directly: 必定会下雨 (it will certainly rain), 她必定很累 (she must surely be very tired). Unlike colloquial alternatives like 肯定, 必定 carries a slightly more objective, almost deductive tone — suggesting the conclusion follows inevitably from known facts or natural laws, rather than personal opinion.