To unfasten means to open, loosen, or disconnect something that was previously closed, tied, or attached. When you unfasten your seatbelt after a car ride, you press the button to release the buckle. When you unfasten your shoelaces, you pull them loose so you can take off your shoes.

The word works for all kinds of closures: you might unfasten the clasp on a necklace, the buttons on a coat, the snaps on a backpack, or the latch on a gate. Each involves undoing something that was holding two things together or keeping something closed.

Unfasten is the opposite of fasten, which means to close or attach. Think of it this way: when you fasten something, you're securing it or making it stay put. When you unfasten it, you're freeing it or opening it back up. A pilot fastens a complicated harness before takeoff, then unfastens it after landing safely.