onesie

Short definition from Wiktionary

Noun

1. 1
   Any of various one-piece attires
2. 2
   Any of various one-piece attires: A one-piece garment for an infant or small child, generally worn over a diaper.
3. 3
   Any of various one-piece attires: One-piece adult loungewear jumpsuit.
4. 4
   Any of various one-piece attires: One-piece fashionable streetwear worn mostly by teenagers.