A coral reef is a type of marine ecosystem composed of living organisms such as coral, fish, and other invertebrates. The three crossword solutions for the term 'coral reef' are ATOLL (5 letters), CAY (3 letters), and KEY (3 letters). An ATOLL is a ring-shaped coral reef that surrounds a lagoon. A CAY is a small, low-lying island made of sand and coral. A KEY is a low-lying island made of coral and limestone. All three of these terms refer to different types of coral reefs.