linguistic unit vs sign

linguistic unit and sign both are nouns.

Word	Noun	Adjective	Verb	Adverb
linguistic unit	Yes	No	No	No
sign	Yes	Yes	Yes	No

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

linguistic unit: one of the natural units into which linguistic messages can be analyzed
sign: a fundamental linguistic unit linking a signifier to that which is signified

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

linguistic unit (noun)	sign (noun)
one of the natural units into which linguistic messages can be analyzed	structure displaying a board on which advertisements can be posted
	a public display of a message
	a gesture that is part of a sign language
	any nonverbal action or gesture that encodes a message
	a character indicating a relation between quantities
	a perceptible indication of something not immediately apparent (as a visible clue that something has happened)
	a fundamental linguistic unit linking a signifier to that which is signified
	an event that is experienced as indicating important things to come
	(astrology) one of 12 equal areas into which the zodiac is divided
	having an indicated pole (as the distinction between positive and negative electric charges)
	(medicine) any objective evidence of the presence of a disorder or disease

linguistic unit (adjective)	sign (adjective)
	used of the language of the deaf

linguistic unit (verb)	sign (verb)
	be engaged by a written agreement
	communicate silently and non-verbally by signals or signs
	mark with one's signature; write one's name (on)
	make the sign of the cross over someone in order to call on God for protection; consecrate
	communicate in sign language
	place signs, as along a road
	approve and express assent, responsibility, or obligation
	engage by written agreement