na-adjective
Describes something that is already decided, fixed, or beyond doubt. Often used for plans, conclusions, or outcomes.
彼の参加は確定的だ。
His participation is definite.
確定的な証拠はまだ
Definite evidence has not been found yet.
確か is a more common, everyday word for 'certain' or 'sure', while 確定的 is more formal and emphasizes a fixed, settled state.
Formed from 確定 (decision, settlement) + 的 (-like, -ish), creating an adjectival noun meaning 'definite' or 'settled'.