Found in the following dictionaries
OTCWL2014 (NASPA Word List 2014)
"Onesie" Definition
noun
A one-piece garment for an infant or small child, generally worn over a diaper.
Source: Wiktionary
A loose-fitting one-piece leisure garment covering the torso and legs:.
Source: Oxford Dictionary
A one-piece close-fitting lightweight garment worn by a young child, usually having sleeves but leaving the legs uncovered and fastening with press studs at the crotch.
Source: Oxford Dictionary