lexeme vs language unit

lexeme and language unit both are nouns.

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

As nouns, language unit is a hypernym of lexeme; that is, language unit is a word with a broader meaning than lexeme:

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'
language unit: one of the natural units into which linguistic messages can be analyzed

Other hypernyms of lexeme include linguistic unit.

lexeme (noun)	language unit (noun)
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'	one of the natural units into which linguistic messages can be analyzed