瞬時|しゅんじshunji[1]atamadaka (head-high)

1. no-adjectiveinstant; moment; (split) second; (in a) flash; (in the) blink of an eye
   コンピューターは、非常ひじょうに複雑ふくざつな仕事しごとを瞬時しゅんじにすることができる。↗

   Computers are capable of doing very complicated work in a split second.

• Synonym瞬間しゅんかんNeutralRefers to a specific moment, as in 「その瞬間」. 「瞬時」 is mostly used adverbially as 「瞬時に」, stressing extreme speed.
• Synonym一瞬いっしゅんNeutralThe everyday word for a split second, as in 「一瞬の出来事」; the most common choice in daily speech.
• Synonym刹那せつなWrittenA Buddhist term for an instant, with a literary feel, as in 「刹那的な生き方」.
• Synonym一瞬間いっしゅんかんWrittenAn emphatic form of 「一瞬」, leaning toward written prose; in speech people say 「一瞬」.
• Synonym瞬刻しゅんこくWrittenMeans an instant but is rare even in writing; 「瞬時」 or 「一瞬」 is the usual choice.
• Synonym一弾指いちだんしWrittenA Buddhist term for the time of a finger snap; rare, with a religious coloring.
• Synonym瞬く間またたくまNeutralThe blink of an eye, mostly as 「瞬く間に」, describing how fast something changes.
• Synonym須臾しゅゆWrittenA moment or short while; archaic and elegant, seen mainly in classical writing.
• Synonym転瞬てんしゅんWrittenIn the blink of an eye; rare even in written prose, mostly as 「転瞬の間に」.
• RelatedモーメントNeutralA loanword for a moment or opportunity; also a physics term for moment of force.
• RelatedとっさNeutralThe moment something happens suddenly, with no time to think, as in 「咄嗟に避ける」 or 「咄嗟に嘘をつく」; 「瞬時」 stresses sheer brevity.
• Antonym永劫えいごうWrittenA written word for eternity, the opposite of the fleeting instant 「瞬時」.
• Antonym長期間ちょうきかんNeutralA long period of time; the opposite of 「瞬時」, which is over in a flash.

Use 「瞬時」 for something done in an extremely short time, as in 「瞬時に判断する」. For an everyday instant say 「一瞬」, for a particular moment 「瞬間」, and for a split-second reaction under pressure 「咄嗟に」. The opposite is 「長期間」.

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

1. 瞬まばたきをする間まほどの僅わずかな時間じかん。