To fasten means to attach, close, or secure something so it stays in place. When you fasten your seatbelt, you click it together so it holds you safely in your seat. When you fasten a button on your shirt, you push it through the buttonhole to keep your shirt closed.

You can fasten things in many different ways: with buttons, zippers, snaps, clips, buckles, ties, or even glue. A carpenter fastens boards together with nails or screws. You might fasten your jacket with a zipper on a cold day, or fasten papers together with a stapler.

When something is fastened, it's held firmly in place and won't easily come loose. A ship fastened to a dock with thick ropes won't drift away. A lid fastened tightly on a jar keeps the contents fresh inside.

The opposite of fasten is unfasten or loosen. When you unfasten your shoes at the end of the day, you're releasing whatever was holding them on your feet, whether that's laces, straps, or buckles.