noun, adverb
Literary or formal word for an extremely short time. Often used with に as an adverbial phrase meaning 'in a flash'.
一刹那の出来事だった。
It happened in a flash.
一刹那に消えた。
It vanished in an instant.
瞬間 is the common, everyday word for 'moment' or 'instant'. 一刹那 is more literary and emphasizes extreme brevity.
From Buddhist terminology, where 一刹那 refers to the shortest unit of time (one thought-moment). The kanji 刹那 is a transliteration of Sanskrit kṣaṇa.