at a glance; by accident
fleeting (glimpse, glance, etc.)
falling lightly (e.g. snow, petals, confetti); fluttering
giving (someone) a peek (of skin, underwear, etc.); giving a glimpse
to glance at; to glimpse
彼女の顔をちらっと見てしまった。
I briefly caught a glimpse of her face.
時計をちらっと見たら遅刻だった。
When I glanced at the clock, I was late.
テストの答えをちらっと見てしまう。
I briefly peeked at the test answer.