A large mass of land surrounded by water is called an island. By definition, an island must be completely surrounded by water but not big enough to be considered a continent.
There are four different types of islands in the world. Continental islands lie on the continental shelves of continents. Such islands include Tasmania, Great Britain and Greenland. Desert islands have no people living on them. Oceanic islands do not sit on continental shelves. Most oceanic islands are volcanic, such as Saint Helena. Tropical islands are the fourth type of island. Examples of tropical islands include Polynesia and the Tioman Islands.