trades union vs trade union

trades union and trade union both are nouns.

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

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

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

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

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