a moment; an instant — especially when a quick reaction is required (often used as the adverb 'とっさに').
車が急に飛び出してきて、とっさにブレーキを踏んだ。
A car suddenly darted out and I braked instinctively.
彼はとっさの判断で人を助けた。
He helped someone with a split-second decision.
とっさに答えられなくて、間が空いてしまった。
I couldn't answer immediately, and there was an awkward pause.
普段から練習していたので、とっさに動けた。
Because I had practiced, I was able to act instantly.