noun, noun which may take the genitive case particle 'no'
Refers to an extremely short period of time, often used in contexts emphasizing speed or suddenness. Can be used as a noun or adjectivally with の.
His expression changed in an instant.
It was a situation that required a split-second decision.
He instantly assessed the situation.
An instant decision is required.
The flames spread in an instant.
一瞬 is a common, everyday word for 'an instant' and is often used adverbially, whereas 瞬時 is more formal and written.
瞬く間 is an idiomatic phrase meaning 'in the blink of an eye,' with a more vivid, narrative feel than the formal noun 瞬時.
瞬時 is a more formal, technical way to say 'an instant' (often in written or scientific contexts), while 咄嗟 carries a everyday, emotional sense of being caught off guard.
instantaneous acquisition model
J-Alert; nationwide instantaneous alert system
Compound of 瞬 (blink, instant) and 時 (time). The exact historical derivation is uncertain, but the kanji combination naturally conveys 'a time as short as a blink'.