relativity n.

early 15c., relatif, "having reference (to something), relating, depending upon," from Old French relatif and directly from Late Latin relativus "having reference or relation," from Latin relatus, used as past participle of referre "bring back, bear back" (see refer), from re- "back, again" + lātus "borne, carried" (see oblate (n.)).

Meaning "having mutual relationship, connected with each other" is from 1590s; that of "arising from or determined by relationship to something else" is from 1610s; that of "having or standing in a relation to something else" is from 1650s; that of "not absolute or existing by itself" is by 1704. In grammar, "referring to an antecedent," from 1520s.

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]