quay vs pier

quay and pier both are nouns.

Word	Noun	Adjective	Verb	Adverb
quay	Yes	No	No	No
pier	Yes	No	No	No

As nouns, pier is a hypernym of quay; that is, pier is a word with a broader meaning than quay:

quay: wharf usually built parallel to the shoreline
pier: a platform built out from the shore into the water and supported by piles; provides access to ships and boats

Other hypernyms of quay include dock, wharf, wharfage.

quay (noun)	pier (noun)
wharf usually built parallel to the shoreline	a platform built out from the shore into the water and supported by piles; provides access to ships and boats
	a support for two adjacent bridge spans
	(architecture) a vertical supporting structure (as a portion of wall between two doors or windows)