A Babygro is a soft stretchy suit that covers a baby's whole body, often including the feet, with poppers down the front or between the legs so you can change a nappy without taking the whole thing off. The name began as a trademark: baby plus grow, because the cloth stretches as the baby gets bigger. In British English many people now use Babygro for any suit of that kind, not only the original brand — rather as some people say Hoover for any vacuum cleaner. Americans more often say onesie or sleeper. It is an everyday parents-and-shops word, a little British in flavour. You would not use it for older children's clothes.