一定 (yīdìng) as an adverb means 'surely, certainly, must' — it expresses strong certainty or a firm intention. Compared to 必须 (bìxū), which emphasizes an external requirement or obligation, 一定 is more about personal determination or subjective certainty. 肯定 (kěndìng) is similar but often used for confident affirmation of a fact, while 一定 can also imply a promise or resolve.