reef vs coral reef

reef and coral reef both are nouns.

Word	Noun	Adjective	Verb	Adverb
reef	Yes	No	Yes	No
coral reef	Yes	No	No	No

As nouns, coral reef is a hyponym of reef; that is, coral reef is a word with a more specific, narrower meaning than reef:

reef (noun)	coral reef (noun)
one of several strips across a sail that can be taken in or rolled up to lessen the area of the sail that is exposed to the wind	a reef consisting of coral consolidated into limestone
a submerged ridge of rock or coral near the surface of the water

reef (verb)	coral reef (verb)
roll up (a portion of a sail) in order to reduce its area
reduce (a sail) by taking in a reef
lower and bring partially inboard