とっさに・咄嗟にtossani

1. adverbat once; immediately; instantly; in an instant; quickly; suddenly; on the spur of the moment

• Synonym直ちにただちにWrittenA formal, written word for acting at once with no delay, as in 「直ちに出発する」; it lacks the nuance of an instinctive reaction in a moment of urgency that 「咄嗟に」 has.
• SynonymすぐにNeutralThe everyday word for 'right away'; it only says the time gap is very short, while 「咄嗟に」 highlights a hurried reaction made with no time to think.
• Synonym即刻そっこくWrittenA solemn, formal word for 'immediately, without a moment to lose'; used in commands as well as narration, as in 「即刻帰宅した」, with no nuance of instinctive reaction.
• Synonym即座にそくざにNeutralDoing something on the spot, right away — for replies as well as actions, as in 「即座に立ち上がる」; it stresses no hesitation, but not necessarily an emergency reaction.
• SynonymすぐNeutralCommon in speech for 'soon, right away'; it can also refer to physical closeness, which 「咄嗟に」 cannot — the latter is about a split-second response.
• SynonymすぐさまNeutralActing immediately, right after something, as in 「すぐさま行動する」; compared with 「咄嗟に」 it focuses more on not wasting time and is not limited to instinctive reactions.
• Synonym即時そくじWrittenA written word for 'instant, in a short time'; it works as a noun or modifier and can describe a person's response, but 「咄嗟に」 stresses a hurried response in a pinch.
• SynonymパッとCasualA colloquial mimetic word for doing something in a flash, as in 「パッと思いつく」; it can also describe something eye-catching, and is not limited to urgent reactions.
• Synonym即そくNeutralAs an adverb it means 'at once'; it also forms compounds like 「即決」「即答」 and can mean 'namely, that is'; it has no nuance of instinctive reaction under pressure.
• Related早速さっそくNeutralSetting about something right away, stressing that you do it actively and of your own accord, as in 「早速始める」 — different from the instinctive 「咄嗟に」.
• Related一瞬いっしゅんNeutralA noun and adverb for 'an instant', as in 「一瞬迷った」; it stresses how short the moment is and carries no sense of an instinctive reaction in an emergency.
• RelatedすかさずNeutralResponding at once, leaving no gap — it can describe a reflex-like reaction; 「咄嗟に」 puts more weight on reacting in a sudden pinch, before there is time to think.
• RelatedたちまちNeutral'In no time' — it describes a situation changing rapidly, as in 「たちまち売り切れる」; the subject is usually a thing or state, not a person's reaction.
• RelatedやにわにWrittenA somewhat old-fashioned word for doing something abruptly, without warning, or on the spot; it stresses the sudden start of the action more than 「咄嗟に」 does.
• RelatedたちどころにWrittenActing at once on the spot, or an effect or result appearing immediately; it stresses that no time is wasted and, unlike 「咄嗟に」, need not be an emergency reaction.
• Related瞬く間にまたたくまにWrittenA written expression meaning 'in the blink of an eye'; it focuses on a change or process taking hardly any time, while 「咄嗟に」 stresses responding at once in a pinch.
• Related時を移さずときをうつさずWrittenA set phrase for doing something without delay; solemn and rare in everyday use, it is about not wasting time and has nothing to do with reactions in an emergency.
• AntonymゆっくりNeutralThe opposite of 「咄嗟に」 in both speed and mindset: doing things slowly and unhurriedly instead of reacting in an instant under pressure.
• Antonym徐々にじょじょにNeutralChanging or progressing gradually, little by little — the opposite of 「咄嗟に」 in timescale, which is about reacting in a single instant.

Use 「咄嗟に」 for an instinctive reaction made in a moment of urgency, before you have time to think, as in 「咄嗟に身をかわす」. To simply say 'right away', choose 「すぐに」; to say you seized the moment and acted straight after something, choose 「すかさず」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 瞬時しゅんじに。即座そくざに。