civilian dress vs plain clothes

civilian dress and plain clothes both are nouns.

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

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

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

Other synonyms of civilian dress include civilian clothing, civilian garb.

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