A surreptitious glance; secret or furtive looking. (Noun; the verb form is 盗み見る.)
彼は彼女の手紙をこっそり盗み見した。
He secretly glanced at her letter.
試験中に隣の答案を盗み見するのは絶対にやめなさい。
Don't ever furtively look at your neighbor's answer sheet during an exam.
子どもがプレゼントの中身を盗み見していたのを見つけた。
I found the child furtively peeking at the contents of the present.