A romper is a piece of clothing consisting of a shirt with attached shorts. Very small children often wear rompers, but sometimes trendy women wear rompers too.
There are adult-sized rompers, most of which are sold as women's clothing, but it's more likely you'll see toddlers wearing rompers. The earliest rompers were worn by children for playing in the early 20th century. They marked a change from the Victorian era, in which kids mainly wore restrictive clothing. In early 1900s France, rompers were considered boys' clothing. Romper comes from romp, "play or frolic."