civilian clothing vs plain clothes

civilian clothing and plain clothes both are nouns.

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

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

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

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

civilian clothing (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.