labor vs labor union

labor and labor union both are nouns.

Word	Noun	Adjective	Verb	Adverb
labor	Yes	No	Yes	No
labor union	Yes	No	No	No

labor (noun)	labor union (noun)
productive work (especially physical work done for wages)	an organization of employees formed to bargain with the employer
any piece of work that is undertaken or attempted
a social class comprising those who do manual labor or work for wages
an organized attempt by workers to improve their status by united action (particularly via labor unions) or the leaders of this movement
concluding state of pregnancy; from the onset of contractions to the birth of a child

labor (verb)	labor union (verb)
undergo the efforts of childbirth
work hard
strive and make an effort to reach a goal