na-adjective
Describes something that happens or changes in an extremely short time, often used with nouns like 変化 (change) or 判断 (judgment).
彼の表情は瞬間的に変わった。
His expression changed momentarily.
瞬間的な判断が求
It was a situation that required an instantaneous decision.
一瞬 is a noun meaning 'an instant' or 'a moment', often used in adverbial phrases like 一瞬で (in an instant). 瞬間的 is more formal and adjectival.
Compound of 瞬間 (moment) + 的 (suffix forming adjectival nouns).