trade union vs trades union

trade union and trades union both are nouns.

Word	Noun	Adjective	Verb	Adverb
trade union	Yes	No	No	No
trades union	Yes	No	No	No

As nouns, trade union and trades union are synonyms defined as:

trade union and trades union: an organization of employees formed to bargain with the employer

Other synonyms of trade union include brotherhood, labor union, union.

trade union (noun)	trades union (noun)
an organization of employees formed to bargain with the employer	an organization of employees formed to bargain with the employer