coast vs seaside

coast and seaside both are nouns.

Word	Noun	Adjective	Verb	Adverb
coast	Yes	No	Yes	No
seaside	Yes	No	No	No

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

Other hyponyms of coast include litoral, littoral, littoral zone, sands, landfall, seaboard, tideland.

coast (noun)	seaside (noun)
the act of moving smoothly along a surface while remaining in contact with it	the shore of a sea or ocean regarded as a resort
the area within view
the shore of a sea or ocean
a slope down which sleds may coast

coast (verb)	seaside (verb)
move effortlessly; by force of gravity