It's a combination of a pauldron (originally part of plate armour) and epaulette (a type of ornamental shoulder piece or decoration used as insignia of rank by armed forces and other organizations).
Hate to be "that guy", but I'm going to anyway: Pictured is a Sandtrooper (a type of stormtrooper). I know a 501st member personally, and he always makes a big deal about the distinction :D
I guess he was a crack master -_-