na-adjective, adverb, noun
inevitably; certainly; without fail
Literary or formal word expressing strong certainty that something will happen; often used in narrative or emphatic statements. Functions as an adverb, adjectival noun, or noun.
この計画は必定成功するだろう。
This plan will certainly succeed.
彼の努力は必定報われるに違いない。
His efforts are bound to be rewarded without fail.
Composed of 必 (certainly) and 定 (fixed, determined). The exact historical derivation is uncertain, but the combination conveys a sense of fixed certainty.