I hit the brakes in a split-second decision.
He reflexively told a lie in that instant.
I instinctively reached out and grabbed the child.
He made a split-second decision to turn the wheel and avoid the accident.
When suddenly asked my name, I couldn't answer right away.
一瞬 simply means 'a brief instant' in terms of duration, whereas 咄嗟 focuses on the unexpectedness that forces an instinctive response.
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.
突然 is an adverb/adjective meaning 'suddenly, out of the blue' describing how an event occurs, while 咄嗟 is a noun describing the split-second moment of reflexive human reaction.
instantly; immediately; on the spur of the moment
Usually written in kana; the kanji form 咄嗟 is rare and may not be recognized by all readers.
Kanji form is uncommon; learners should prefer the kana spelling.
A Sino-Japanese word written 咄嗟. 咄 and 嗟 represent brief exclamations or tongue-clicking; from the idea of the moment needed to utter them, the compound developed the sense “instant” or “split second.”