capability (n.)

"sufficiently able, having power or capacity, qualified," 1590s, from French capable "able, sufficient; able to hold," or directly from Late Latin capabilis "receptive; able to grasp or hold," used by theologians, from Latin capax "able to hold much, broad, wide, roomy;" also "receptive, fit for;" adjectival form of capere "to grasp, lay hold, take, catch; undertake; take in, hold; be large enough for; comprehend" (from PIE root *kap- "to grasp"). Other late 16c. senses in English, now obsolete, were "able to comprehend; able to contain; extensive." Related: Capably.

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]