individuality n.

early 15c., "one and indivisible, inseparable" (with reference to the Trinity), from Medieval Latin individualis, from Latin individuus "indivisible," from in- "not, opposite of" (see in- (1)) + dividuus "divisible," from dividere "divide" (see divide (v.)). Original sense now obsolete; the word was not common before c. 1600 and the 15c. example might be an outlier. Sense of "single, separate, of but one person or thing" is from 1610s; meaning "intended for one person" is from 1889.

Individual views a person as standing alone, or persons as standing separately before the mind: as, the rights of the individual; the rights of individuals: it is incorrect to use individual for person unemphatically ; as, there were several individuals in the room. [Century Dictionary]

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]