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.
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.
The distinction between a jumper dress and an apron dress is unclear, the terms often being used interchangeably. Typically, if the design of the dress is directly inspired by an apron (having a bib in front and ties in the back, for example), the garment is described as an apron dress (note these pattern examples: early 1950's, 1974, and an unknown time period. Compare with this magazine excerpt and jumper dress patterns: women's magazine Paris Modes (Sept. 1909), 1940, 1956, 1960.
To confuse matters, a Viking Apron Dress (admittedly a construction based on conjecture from the Hedebyfragments) most resembles the 1909 jumper dress and not the 1950's apron dress.
See also
Gymslip (British term for a pinafore dress worn as athletic wear or school uniform)
