key word vs word

key word and word both are nouns.

Word	Noun	Adjective	Verb	Adverb
key word	Yes	No	No	No
word	Yes	No	Yes	No

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

key word (noun)	word (noun)
a significant word used in indexing or cataloging	a unit of language that native speakers can identify
a word that is used as a pattern to decode an encrypted message	a promise
	information about recent and important events
	a brief statement
	an exchange of views on some topic
	a secret word or phrase known only to a restricted group
	a verbal command for action
	a string of bits stored in computer memory

key word (verb)	word (verb)
	put into words or an expression