Oops , sorry, regional term. Bonds
is a very popular Australian brand, baby clothes, t shirts, underwear, all sorts of knits. The most famous Bonds item is the Jackie How singlet, blue and worn by shearers and other hard workers (for about the last 80 years and still popular) - you may have seen it on Paul Hogan in Crocodile Dundee. This singlet vies with the Akubra hat and the Drizabone coat as Australian national dress. Bonds is now owned by Pacific Brands and apparently the underwear waist band can now be seen sticking out from the top of American boys' falling down jeans and is a "designer" name.
. This is sort of funny to everyone over 30 whose mother made them wear sensible bonds singlets and whose grandmothers wore "cottontails", aka granny knickers. (All my children have to wear sensible Bonds singlets and knickers......)
To get back on topic
where I live these are called bonds suits, but where my sister lives they are called grow-suits, and my MIL and her friends call these "bunny suits" When I first visited America 14 or so years ago I did not understand the term "onesies" but now my (Australian) SIL's both use the term - but for t shirts with an extended buttoning crotch. Apparently when there are legs the term is different here.