plain clothes vs civilian dress

plain clothes and civilian dress both are nouns.

Word	Noun	Adjective	Verb	Adverb
plain clothes	Yes	No	No	No
civilian dress	Yes	No	No	No

As nouns, plain clothes and civilian dress are synonyms defined as:

plain clothes and civilian dress: ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.

Other synonyms of plain clothes include civilian clothing, civilian garb.

plain clothes (noun)	civilian dress (noun)
ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.	ordinary clothing as distinguished from uniforms, work clothes, clerical garb, etc.