accessibility (n.)

c. 1400, "affording access, capable of being approached or reached," from Old French accessible and directly from Late Latin accessibilis, verbal adjective from Latin accessus "a coming near, an approach; an entrance," from accedere "approach, go to, come near, enter upon" (see accede). The meaning "easy to reach" is from 1640s; of art or writing, "able to be readily understood," by 1961 (a word not needed before writing or art often deliberately was made not so). Related: Accessibility.

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]