A coil is something wound in a circular or spiral shape, like a snake curled up or a rope twisted round and round. When you take a garden hose and wind it into loops before storing it, you're coiling it. A spring inside a pen is a tight coil of metal wire. Your hair might form natural coils, or you might coil a long extension cord to keep it from tangling.

The shape matters because coiling serves practical purposes. Sailors coil rope on ships so it doesn't become a messy tangle. Snakes coil their bodies to rest, stay warm, or prepare to strike. In science, coils of wire become electromagnets when electricity flows through them: the circular shape creates a magnetic field that helps electric motors, speakers, and generators work.

You can also use coil as a verb. A cat might coil itself into a tight ball for a nap. Smoke coils upward from a campfire in lazy spirals. The key idea is that circular, wound-up shape, whether it's something you create deliberately or something that forms naturally as it moves or rests.