To peek or peep through a keyhole, gap, or other small opening (to look secretly or furtively).
子供がドアの隙間から部屋の中を覗いていた。
A child was peeking into the room through the gap in the door.
鍵穴に顔を寄せて中を覗くのは危険だ。
Putting your face up to a keyhole to peek inside is dangerous.
彼はこっそりカーテンの隙間を覗いた。
He quietly peeked through the gap in the curtain.
To look down into a ravine, hole, or other deep place (to peer downward into something).
展望台から谷の底を覗くと、小さな川が見えた。
When I peered down from the observatory, I could see a small river at the bottom of the valley.
工事現場の深い穴を覗くのは禁止されている。
Peering into the deep pit at the construction site is prohibited.
To peek into a shop, bookstore, etc., from the outside (to take a quick look inside a place).
新しいカフェができていたので、通りから中を覗いてみた。
A new café had opened, so I peeked inside from the street.
ウィンドウを覗いて靴を選んだ。
I looked into the shop window to choose shoes.
To sneak a quick look; to take a brief or furtive glance.
試験の合格発表をちらっと覗いただけで、まだ喜べない。
I only took a quick peek at the exam results, so I can't celebrate yet.
彼女は相手の顔を一瞬覗いて、すぐ目をそらした。
She glanced at the other person's face for a moment and immediately looked away.
To peep into or look through instruments such as microscopes or telescopes (to observe using optical devices).
顕微鏡を覗いて細胞の構造を確認した。
I looked through the microscope to check the structure of the cells.
天体望遠鏡で月を覗くと、クレーターがはっきり見えた。
When I looked at the moon through the telescope, the craters were clearly visible.
To stick out slightly or to be visible through a gap (intransitive); e.g., a scarf peeking from a collar, the sky peeking through trees.
セーターの襟元から白いシャツが覗いている。
A white shirt is peeking out from the sweater's collar.
山の間から朝日が覗き始めた。
The morning sun began to peek out between the mountains.
To examine or study someone's expression or face closely (to read their mood or intentions).
医者は患者の顔色を覗いて、具合を確かめた。
The doctor looked closely at the patient's complexion to check how they were doing.
面接官が応募者の目を覗いて反応を見た。
The interviewer studied the applicant's eyes to gauge their reaction.
Archaic: to face or to be positioned toward something (an old usage found in classical texts).
（古文）城門に覗きて兵を整う。
(classical) Facing the castle gate, the soldiers prepared.