Sun dress in cotton fabric is a very light dress, suitable for wearing outdoors or on beaches in bright sunshine. There is an inner lining in thin cotton fabric that adds to the comfort. Machine embroidery has also been done impart the dress some more attraction. A sundress, like a jumper, is sleeveless and collarless. However it isn't worn over a blouse or sweater, and is of a distinctly different cut and fashion.
In British English, the term jumper describes a sweater. Also, in more formal British usage, a distinction is made between a pinafore dress and a pinafore, which, though a related garment, has an open back and is worn as an apron. Pinny is a shorter name for both garments.