"reflectiveness, degree to which a thing or surface reflects or is reflected," 1849, from reflective + -ity.
Entries linking to reflectivity
reflective adj.
1620s, "throwing back rays or images, giving reflections of objects, reflecting," from reflect + -ive. From 1670s as "of or pertaining to (mental) reflection, taking cognizance of the operations of the mind." By 1820 as "having a tendency to or characterized by (mental) reflection, meditative, thoughtful." Related: Reflectively; reflectiveness.
-ity
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]