An instant or moment; a very brief point or interval of time. Used as a noun or adverb to describe something happening in a flash.
一瞬、目の前がまぶしくなった。
For an instant, everything in front of me went bright.
彼の表情は一瞬で変わった。
His expression changed in an instant.
その映像は一瞬しか映らなかったが、強い印象を残した。
The image only appeared for a moment, but it left a strong impression.