A chemise is a simple, loose-fitting dress. If you're heading to the beach, you might throw a chemise on over your bathing suit.
Use the word chemise for an unstructured garment without any defined waist — think of the dresses flappers wore in the 1920s, which hung straight down from their shoulders. It's just as common to call a nightgown or a slip — a type of undergarment worn beneath a dress — a chemise. The word comes from Old French, meaning "tunic," and was originally used for the tunics worn by soldiers beneath their armor.