- a one-piece leisure or sleeping garment for an adult, usually combining a long-sleeved top with long pants.
- a brand name for one-piece garments for infants, the lower portion resembling briefs and having snaps across the crotch for convenience in diapering. Although a trademarked term, it is often spelled as lowercase onesie, referring to a single garment.
- a one-piece garment combining a top with trousers, worn by adults as leisurewear
