An Instant
A Moment
noun
One blink is one instant. That is an instant or a moment.
いっしゅん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
「お前には捕まらないぞ！」「捕まえたっ！」「うわっ！速い！一瞬だったな。でも、大人げないよ。子どもなんだからもう少し手加減してよ。」
"You’ll never catch me!" "Gotcha!" "What! That was fast! You caught me in an instant, but that’s childish. You should go a little easier on your kid."