synonymity vs synonymousness

synonymity and synonymousness both are nouns.

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

As nouns, synonymity and synonymousness are synonyms defined as:

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

Other synonyms of synonymity include synonymy.

synonymity (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