by-blow vs illegitimate child

by-blow and illegitimate child both are nouns.

Word	Noun	Adjective	Verb	Adverb
by-blow	Yes	No	No	No
illegitimate child	Yes	No	No	No

As nouns, by-blow and illegitimate child are synonyms defined as:

by-blow and illegitimate child: the illegitimate offspring of unmarried parents

Other synonyms of by-blow include bastard, illegitimate, love child, whoreson.

by-blow (noun)	illegitimate child (noun)
the illegitimate offspring of unmarried parents	the illegitimate offspring of unmarried parents