An instant; a very short moment or split second (often used as 瞬時に to mean 'instantly').
光は瞬時に広がった。
Light spread in an instant.
彼は問題の原因を瞬時に見抜いた。
He saw through the cause of the problem in an instant.
瞬時に判断を下すのは難しい。
It's difficult to make a decision instantly.
痛みは瞬時に消えた。
The pain disappeared instantly.