The main difference between Cay and Island is that the Cay is a small island formed on the surface of a coral reef and Island is a sub-continental land that is surrounded by water.
A cay ( or ), also spelled caye or key, is a small, low-elevation, sandy island on the surface of a coral reef. Cays occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including in the Caribbean and on the Great Barrier Reef and Belize Barrier Reef).
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be called an eyot or ait, and a small island off the coast may be called a holm. A grouping of geographically or geologically related islands is called an archipelago, such as the Philippines, for example.
An island may be described as such, despite the presence of an artificial land bridge; examples are Singapore and its causeway, and the various Dutch delta islands, such as IJsselmonde. Some places may even retain “island” in their names for historical reasons after being connected to a larger landmass by a land bridge or landfill, such as Coney Island and Coronado Island, though these are, strictly speaking, tied islands. Conversely, when a piece of land is separated from the mainland by a man-made canal, for example the Peloponnese by the Corinth Canal or Marble Hill in northern Manhattan during the time between the building of the United States Ship Canal and the filling-in of the Harlem River which surrounded the area, it is generally not considered an island.
There are two main types of islands in the sea: continental and oceanic. There are also artificial islands.
A small, low island largely made of sand or coral.
A contiguous area of land, smaller than a continent, totally surrounded by water.
An entity surrounded by other entities that are very different from itself.
“an island of tranquility (a calm place surrounded by a noisy environment)”
“an island of colour on a butterfly’s wing”
A superstructure on an aircraft carrier’s deck.
A traffic island.
“the island in the middle of a roundabout”
An unincorporated area wholly surrounded by one or more incorporated areas.
A phrase from which a wh-word cannot be extracted without yielding invalid grammar.
To surround with water; make into an island.
To set, dot (as if) with islands.