A babydoll is a very short, loose-fitting negligee. It is typically sleeveless, but may have very short "cap" sleeves. Within this category, there are endless variations. A babydoll may or may not cover the buttocks. It may feature defined bra cups or hang loosely. Fabric choices range from silk or satin to lace or sheer fabrics.
Due to loose fitting nature of baby doll lingerie, it is easy to determine what size to buy. Babydolls are generally sized the same way as dresses (6, 8, 10 etc). Buy a babydoll in the same size as the dresses your partner wears.