synonymy vs synonymousness

synonymy and synonymousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
synonymy	Yes	No	No	No
synonymousness	Yes	No	No	No

As nouns, synonymy and synonymousness are synonyms defined as:

synonymy and synonymousness: the semantic relation that holds between two words that can (in a given context) express the same meaning

Other synonyms of synonymy include synonymity.

synonymy (noun)	synonymousness (noun)
the semantic relation that holds between two words that can (in a given context) express the same meaning	the semantic relation that holds between two words that can (in a given context) express the same meaning