An islet is a small island. A rock, a sometime synonym for a type of "islet", is a landform comprised of rock, lying offshore, having no or minimal vegetation, and uninhabited (see Mōkōlea Rock at left).
An exposed sandbar would be another type of islet. A more technical application is to small land features, isolated by water, laying off the shore of a larger island. And in related fashion, any emergent land on an atoll (a type of island) would also be called an islet.