pier vs quay

pier and quay both are nouns.

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

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

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

Other hyponyms of pier include levee.

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