What Does "Key" Mean in "Florida Keys"?

In this context, "key" (from Spanish cayo) means a small, low-elevation island or islet formed from coral reefs, sand, or sediment—not volcanic rock.

The Florida Keys are a coral cay archipelago curving ~110 miles from Key Largo (north) to Key West (south), dividing the Atlantic Ocean from the Gulf of Mexico. They are the exposed portions of an ancient coral reef tract.

Early names: Los Mártires (Ponce de León), Cayo Hueso (Key West = "Bone Key"), Cayos de las Tortugas (Turtle Keys). English sailors adopted "Keys" by the 1700s.