fastness vs fixity

fastness and fixity both are nouns.

Word	Noun	Adjective	Verb	Adverb
fastness	Yes	No	No	No
fixity	Yes	No	No	No

As nouns, fastness and fixity are synonyms defined as:

fastness and fixity: the quality of being fixed in place as by some firm attachment

Other synonyms of fastness include fixedness, fixture, secureness.

fastness (noun)	fixity (noun)
a strongly fortified defensive structure	the quality of being fixed in place as by some firm attachment
a rate (usually rapid) at which something happens	the quality of being incapable of mutation
the quality of being fixed in place as by some firm attachment