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1580s, "reflective, capable of bending or turning back," from Medieval Latin reflexivus, from Late Latin reflexus (see reflect). Meaning "of the nature of a reflex" is from 1839 (implied in reflexively). Grammatical sense from 1837. Related: Reflexiveness; reflexivity.
Of or relating to a mathematical or logical relation such that, for any given element, that element has the given relation to itself. Equality in mathematics is a reflexive relation, since a = a for all a, whereas the relation of being 'less than' is not, since it is not true that a < a for any a.