language unit vs lexeme

language unit and lexeme both are nouns.

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

As nouns, lexeme is a hyponym of language unit; that is, lexeme 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
lexeme: a minimal unit (as a word or stem) in the lexicon of a language; `go' and `went' and `gone' and `going' are all members of the English lexeme `go'

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

language unit (noun)	lexeme (noun)
one of the natural units into which linguistic messages can be analyzed	a minimal unit (as a word or stem) in the lexicon of a language; `go' and `went' and `gone' and `going' are all members of the English lexeme `go'