A rash guard, also known as rash vest or rashie, is an athletic shirt made of spandex and nylon or polyester. The name rash guard reflects the fact that the shirt protects the wearer against rashes caused by abrasion, or by sunburn from extended exposure to the sun.
A rash guard shirt is usually worn by itself when surfing in weather too warm for a wetsuit to prevent chafing from sliding on and off of the waxed surface of the surf board. A surfboard's wax holds sand from the beach that can rub against a surfer while paddling out to the break, or legs while sitting atop one's board.
Rash guards also offer some protection from the sun (measured by its Ultraviolet Protection Factor) and slight protection against jelly fish stings and are sometimes worn under wetsuits to prevent chafing. A rash guard helps to prevent irritation caused by rapid impact with surface water and waves as well.