"Onesie" is the word most people reach for, but on this site it covers two genuinely different pieces, and knowing which one you actually need saves a lot of confused browsing. A bodysuit is a snug, short-bodied piece that snaps at the nappy and is designed to be worn under other clothes, not as a full outfit by itself. It is the base layer, not the outer layer.
A romper, by contrast, is a complete outfit with shorts or a short leg built in, meant to be worn on its own. So when someone searches for an infant green onesie, they are usually picturing exactly what we would call a bodysuit: a simple, snug base layer in a single colour that works under dungarees, cardigans or dresses without adding bulk.
The confusion is understandable because both are one-piece garments with a snap crotch. The practical test is simple: if it is meant to go under something else, it is a bodysuit. If it is meant to be the whole outfit, it is a romper or jumpsuit.
If you are building a newborn wardrobe and unsure which to buy first, bodysuits are the more essential piece. They layer under almost everything else in the wardrobe, while rompers and dresses only work on their own.