language unit vs word

language unit and word both are nouns.

Word	Noun	Adjective	Verb	Adverb
language unit	Yes	No	No	No
word	Yes	No	Yes	No

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

language unit: one of the natural units into which linguistic messages can be analyzed
word: a unit of language that native speakers can identify

Other hyponyms of language unit include discourse, syllable, lexeme, morpheme, formative, name, collocation, phone, sound, speech sound, sign.

language unit (noun)	word (noun)
one of the natural units into which linguistic messages can be analyzed	a unit of language that native speakers can identify
	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

language unit (verb)	word (verb)
	put into words or an expression