The following article was sourced from a Wikipedia page at the following address: http://en.wikipedia.org/wiki/Jumper_(dress)
A jumper (in American English), pinafore dress or pinafore (British English) is a sleeveless, collarless dress intended to be worn over a blouse, shirt or sweater. In British English, the term jumper describes what is called a sweater in American English. Also, in more formal British usage, a distinction is made between a pinafore dress and a pinafore. The latter, though a related garment, has an open back and is worn as an apron. In American English, a pinafore always refers to an apron.
A sundress, like a jumper, is sleeveless and collarless; however, such articles are not worn over a blouse or sweater, and are of distinctly different cuts and fashions. The apron dress may be viewed as a special case of the jumper. 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 typically described as an apron dress.
The Viking Apron Dress (admittedly a construction based on conjecture from the Hedeby fragments), is of a design that is in line with a standard jumper. The name was most likely chosen owing to the dress sometimes being worn on top of a sleeved dress.
To read more about jumpers (pinafores), please click on the following link: http://en.wikipedia.org/wiki/Jumper_(dress)