Vagina: The muscular canal that extends from the cervix to the outside of the body. It is usually 6 to 7 inches in length, and its walls are lined with mucous membrane. It includes two vaultlike structures: the anterior (front) vaginal fornix and the posterior (rear) vaginal fornix. The cervix protrudes slightly into the vagina, and through a tiny hole in the cervix (the os), sperm make their way toward the internal reproductive organs. The vagina also includes numerous tiny glands that make vaginal secretions.