The protective shirt for swimming/surfing has many aliases. Originally the rash guard was designed to protect surfers chests from surfboard or boogie board rashes but with increasing awareness (of the importance of sun protection) the rash guard evolved into a staple part of one's swimwear wardrobe.
Referred to by a variety of names, depending on which part of the world you are from, there are a few distinguishing features but, most simply, the rash guard is tight fitting and a sun or swim shirt is looser fitting. At Surlaplage we have the 'rash shirt', which is designed to be slim fitting. To make life easier, the Aussies have simplified it and put all sun protective tops under one umbrella. They are affectionately referred to as 'rashies'. If you know of any other names please add them to the comment section below.