reflex (n.)

c. 1500, "reflection of light, image produced by reflection," from a verb reflex meaning "refract, deflect" (late 14c.; compare reflect), from Late Latin reflexus "a bending back," noun use of past participle of reflectere "to bend back, bend backwards, turn away," from re- "back" (see re-) + flectere "to bend" (see flexible). Also as an adjective (1640s), "thrown or turned backward," also of thoughts or the mind. Meaning "involuntary nerve stimulation" is recorded by 1877, short for reflex action (1833) "simple, involuntary action of the nervous system."

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Definitions of reflex from WordNet
reflex (n.)
an automatic instinctive unlearned reaction to a stimulus;
Synonyms: reflex response / reflex action / instinctive reflex / innate reflex / inborn reflex / unconditioned reflex / physiological reaction
reflex (adj.)
without volition or conscious control;
a reflex knee jerk
Synonyms: automatic / reflexive