1. a one-piece garment for a baby or young child, typically with long sleeves and legs and a snap or button closure at the crotch
Usage: informal; common in North American English
2. a one-piece pajama or costume garment worn by older children or adults, often designed to resemble an animal or character
Usage: informal; popular as sleepwear or costume

Examples

• The baby wore a soft cotton onesie with little stars printed on it.
• She packed several onesies for the newborn's hospital bag.
• The kids wore animal onesies to the pajama party.
• He bought a dinosaur onesie as a funny gift for his friend.
• Onesies are convenient for babies because they stay tucked in and don't ride up.
• Many parents prefer onesies because they are easy to change and keep babies warm.