As nouns the difference between sleepers and onesie
is that sleepers is while onesie is (us) a one-piece garment for an infant or small child, generally worn over a diaper.
onesie
English
Noun
(
en noun)
(US) A one-piece garment for an infant or small child, generally worn over a diaper.
One-piece adult loungewear jumpsuit.
One-piece fashionable streetwear worn mostly by teenagers.
Synonyms
* (a one-piece baby garment) romper, romper suit