noun, noun which may take the genitive case particle 'no'
Refers to a very short moment, often used in adverbial phrases like とっさに (in an instant, reflexively) or とっさの (instantaneous, spur-of-the-moment).
See also: 咄嗟に (とっさに)
とっさの判断でブレーキを踏んだ。
I hit the brakes in a split-second decision.
彼はとっさ
He reflexively told a lie in that instant.
瞬間 is a more general and common word for 'moment' or 'instant', while とっさ emphasizes a sudden, reflexive, or unexpected moment.
The origin is uncertain; it may be a sound-symbolic word mimicking suddenness. The kanji 咄嗟 are ateji (characters used for their sound rather than meaning).