A duster is a light, loose-fitting long coat.
The original dusters were full-length, light-colored canvas or linen coats worn by horsemen to protect their clothing from trail dust. These dusters were typically slit up the back to hip level for ease of wear on horseback and were the recommended "uniform" for Texas Rangers. Dusters intended for riding may have features such as a buttonable rear slit and leg straps to hold the flaps in place. For better protection against rain, dusters were made from oilcloth and later from waxed cotton.
Dusters today 
Western horsemen's dusters gained renewed popularity in the late twentieth century and are now a standard item of western wear. They figured little in Western films until Sergio Leone re-introduced them in his movies The Good, the Bad, and the Ugly and Once Upon a Time in the West. The latter played for many months in Paris and was in part credited with a revival of the duster in men's fashions in that city. In modern times, leather dusters are worn by motorcyclists to prevent road rash.
See also 
- Picken, Fashion Dictionary
- George-Warren, Holly, and Michelle Freedman: How the West Was Worn, Harry N. Abrams (2001), ISBN 0-8109-0615-5.
- Picken, Mary Brooks: The Fashion Dictionary, Funk and Wagnalls, 1957. (1973 edition ISBN 0-308-10052-2)
- Merriam Webster's 10th Collegiate Dictionary
- Oxford English Dictionary