- 1一瞬間、一剎那；當形容詞是「瞬間的、分秒不差的」（如：瞬間的決定、精準到毫秒的時機）
A split second is an extremely short moment — less than a second, so short you could not really measure it. 'For a split second I thought it was my .' You use it for something that happened almost too fast to notice, or for a moment of thought or feeling that came and went instantly. It is also used as an adjective before a noun, and there it means 'made or done in that tiny moment, and needing to be exactly right': a split-second decision, split-second timing. That adjective often carries a little admiration — it says the timing was so fine that being slightly late would have ruined it. The phrase came from a kind of with two hands, one of which could be stopped on its own to catch an in-between time; the second hand was 'split'. As a noun it is usually written as two words, and as an adjective before a noun, with a .
split second 是極短的一瞬間，短到幾乎無法真正量出來。它可以用來形容快到幾乎沒察覺的事，例如「For a split second I thought it was my brother」（有那麼一瞬間，我以為那是我弟弟），也可以指一閃而過的念頭或感覺。
放在名詞前面當形容詞時，意思會變成「在那一瞬間完成、而且必須非常精準的」，例如 a split-second decision（瞬間做出的決定）、split-second timing（分秒不差的時機）。這種說法常帶一點讚賞，表示時機抓得太精準，晚一點點就會全盤皆輸。
這個說法的來源，是一種有兩根指針的碼錶，其中一根可以單獨停下來，抓住中間的時間，所以秒針被「分開」了。寫法上，名詞通常寫成兩個字，形容詞放在名詞前時要加連字號。
From the word’s history
Compound of split + second, from split-seconds hands on a stopwatch.
由 split 加 second 組成，源自碼錶上的「分段秒針」。
組成split（分開）+ second（秒）。Literally a second cut into pieces — from an old stopwatch whose second hand could be split in two to catch a time.
原始詞典資料2 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
split-secondnoun/ˌsplɪtˈsɛk(ə)nd/
- 1
A stopwatch with two second hands, each of which may be stopped separately to time the intervals of a race by a sportsperson, or to time more than one sportsperson.
- 2
A fraction of a second; hence (figurative, also attributive) a very short time period; an instant, a moment.
- It's very dangerous, though, when a man thinks for a split second that he understands death.
- I was in the gutter, sitting my bike, willing them to die, […] or otherwise obliterated because this demanded split-second timing.
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。