early 15c., of situations or qualities, "great, extreme," from Old French intense (13c.), from Latin intensus "stretched, strained, high-strung, tight," originally past participle of intendere in its literal sense of "stretch out, strain" (see intend). From 1630s of persons, "high-strung." Related: Intensely; intenseness.
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]
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Definitions of intensity from WordNet
the amount of energy transmitted (as by acoustic or electromagnetic radiation);