What is an island?

"Although “island” tends to conjure images of small bodies of land surrounded by water, such as Bermuda, or the Falkland Islands, this is not the only kind of island out there. Lakes are islands of water surrounded by land. Caves are islands of darkness surrounded by light. Oases are islands of fertility surrounded by sand. In short, an island is any self-contained patch of habitat within some larger sea. Looked at this way, the garden outside my window is an island of parkland in an ocean of bricks and concrete."

References:
Divide and Diminish. NYTimes.