positivity n.

early 14c., originally a legal term meaning "formally laid down, decreed or legislated by authority" (opposed to natural),  from Old French positif (13c.) and directly from Latin positivus "settled by agreement, positive" (opposed to naturalis "natural"), from positus, past participle of ponere "put, place" (see position (n.)).

The sense of "absolute" is from mid-15c. Meaning in philosophy of "dealing only with facts" is from 1590s. Sense broadened to "expressed without qualification" (1590s), then, of persons, "confident in opinion" (1660s). The meaning "possessing definite characters of its own" is by 1610s. The mathematical use for "greater than zero" is by 1704. Psychological sense of "concentrating on what is constructive and good" is recorded from 1916. Positive thinking is attested from 1953. The sense in electricity is from 1755.  

There are probably no two bodies differing in nature which are not capable of exhibiting electrical phaenomena, either by contact, pressure, or friction ; but the first substances in which the property was observed, were vitreous and resinous bodies ; and hence the different states were called states of resinous and vitreous electricity ; and resinous bodies bear the same relation to flint glass, as silk. The terms, negative and positive electricity, have been likewise adopted, on the idea, that the phaenomena depend upon a peculiar subtile fluid, which becomes in excess in the vitreous, and deficient in the resinous bodies ; and which is conceived by its motion and transfer, to produce the electrical phaenomena. [Sir Humphry Davy, "Elements of Chemical Philosophy," London, 1812]

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]