A onesie is a comfortable piece of clothing that covers the body, the arms, and the legs in one piece — no separate shirt and trousers. You pull it on (or zip it up) and you are dressed.

The name started as a brand: Gerber called baby suits Onesies, and people began using onesie for the thing itself. For a baby, it is often a soft cotton suit with snaps at the bottom, so you can change a nappy without taking the whole thing off. For an adult, it is usually loungewear: a zip-up suit you might wear on the sofa all Sunday, sometimes with a hood, sometimes made to look like an animal.

Though actually, shops are messy here. In some places a baby "onesie" is only a bodysuit: it covers the body and arms, but the legs are bare. The long version that covers the legs may be called a sleeper or footie instead. The dictionary picture — arms and legs all in — is truest of the adult suit and of long baby ones. The word is informal.