A gown is a formal dress. When your date to a party or dance is wearing a tuxedo, you'll probably want to wear a gown.
There are many different kinds of gowns, including ball gowns, wedding gowns, and evening gowns. All of them are worn primarily by women on formal occasions. It was once common to call any one-piece woman's garment a gown, while now it's more often called a dress. The source of gown is the Old French goune, "robe, coat, or down," with the Latin root gunna, "hide or skin."