A onesie is a one-piece, t-shirt-like baby garment that has snaps at the crotch to keep it from riding up and exposing the baby's tummy. Onesies are also called bodysuits. The term "onesie" is most often used to describe a short-sleeved bodysuit that does not cover the baby's legs, but it also can refer to versions with long sleeves and long legs. Onesies can be used as a complete outfit, as a t-shirt, or as an undergarment. Onesies range in style from simple, white, undergarment versions to colorful, patterned and embroidered types.

a one-piece baby garment that resembles a t-shirt and has snaps at the crotch

a one-piece outfit for a baby

a piece of baby clothing that covers the upper body and snaps between the legs, making it easy to change diapers.

This is a one piece “shirt” that buttons up at the waist and goes from baby's shoulders around the bottom. Generally worn like an undershirt though they make great summer pajamas when it's really hot! (See Diaper Vest)

Onesie or onesize is American English for a bodysuit designed to conceal a diaper when worn. This one-piece garment is sometimes also referred to as a creeper or diaper shirt. The shirt extends past the waist and has snaps or buttons that allow it to be closed over the crotch, with enough extra room to accommodate a diaper.