availability (n.)

mid-15c., "beneficial," also "valid, effective, capable of producing the desired effect," from avail + -able. The meaning "at one's disposal, capable of being made use of" is recorded from 1827. Related: Availably.

1855, from un- (2) "opposite of" + availability, or else from unavailable + -ity.

word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).

Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]