A cay (also spelled key or quay; pronounced as "key" IPA: /kiː/) is a small, low-elevation, sandy island formed on the surface of coral reefs. 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), where they provide habitable and agricultural land for hundreds of thousands of people. Their surrounding reef ecosystems also provide food and building materials for island inhabitants.

The Cay is a novel written by Theodore Taylor. The story is based on a real incident recounted to Taylor. The novel was published in 1969.

Çay is a town and a district of Afyonkarahisar Province in the Aegean region of Turkey.