The Collaborative International Dictionary of English v.0.48:

Reflex \Re"flex\ (r?"fl?ks; formerly r?*fl?ks"), n. [L. reflexus
   a bending back. See Reflect.]
   1. Reflection; the light reflected from an illuminated
      surface to one in shade.
      [1913 Webster]

            Yon gray is not the morning's eye,
            'Tis but the pale reflex of Cynthia's brow. --Shak.
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            On the depths of death there swims
            The reflex of a human face.           --Tennyson.
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   2. (Physiol.) An involuntary movement produced by reflex
      action.
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   Patellar reflex. See Knee jerk, under Knee.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Reflex \Re*flex"\ (r?*fl?ks"), v. t. [L. reflexus, p. p. of
   reflectere. See Reflect.]
   1. To reflect. [Obs.] --Shak.
      [1913 Webster]

   2. To bend back; to turn back. --J. Gregory.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Reflex \Re"flex\ (r?"fl?ks), a. [L. reflexus, p. p. of
   reflectere: cf. F. r['e]flexe. See Reflect.]
   1. Directed back; attended by reflection; retroactive;
      introspective.
      [1913 Webster]

            The reflex act of the soul, or the turning of the
            intellectual eye inward upon its own actions. --Sir
                                                  M. Hale.
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   2. Produced in reaction, in resistance, or in return.
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   3. (Physiol.) Of, pertaining to, or produced by, stimulus or
      excitation without the necessary intervention of
      consciousness.
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   Reflex action (Physiol.), any action performed
      involuntarily in consequence of an impulse or impression
      transmitted along afferent nerves to a nerve center, from
      which it is reflected to an efferent nerve, and so calls
      into action certain muscles, organs, or cells.

   Reflex nerve (Physiol.), an excito-motory nerve. See
      Exito-motory.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reflex
    adj 1: without volition or conscious control; "the automatic
           shrinking of the pupils of the eye in strong light"; "a
           reflex knee jerk"; "sneezing is reflexive" [syn:
           automatic, reflex(a), reflexive]
    n 1: an automatic instinctive unlearned reaction to a stimulus
         [syn: reflex, reflex response, reflex action,
         instinctive reflex, innate reflex, inborn reflex,
         unconditioned reflex, physiological reaction]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "reflex":
   Pavlovian conditioning, action and reaction, adumbration, answer,
   antiphonal, arch, automatic, automatic reaction,
   automatic response, autonomic reaction, back, backlash, backward,
   backwash, behavior, bend, bend back, blind, blind impulse,
   bounceback, bow, brain wave, brainstorm, casual, clout, compulsive,
   conditioned, conditioned response, conditioning, counter, crook,
   curl, curve, decurve, deflect, dome, drive, echo, embow, fancy,
   flash, fleeting impulse, flex, force, forced, gut, gut response,
   hook, hump, hunch, ill-advised, ill-considered, ill-devised,
   impact, impress, impression, imprint, impulse, impulsive,
   inadvertent, incurvate, incurve, indeliberate, inflect,
   inspiration, instinct, instinctive, instrumental conditioning,
   involuntary, involuntary impulse, loop, mark, mechanical,
   natural impulse, negative reinforcement, notion,
   operant conditioning, outline, positive reinforcement,
   predictable response, print, psychagogy, quick hunch, reacting,
   reaction, reactionary, reactive, recoil, recurve, reeducation,
   reflect, reflection, reflex action, reflexive, refluence, refluent,
   reflux, reinforcement, reorientation, repercussion, reply,
   respondence, respondent, responding, response, responsive,
   retroaction, retroactionary, retroactive, retroflex, return,
   reverberation, reversed, revulsion, revulsive, rise, round, sag,
   shadow, silhouette, snap, spontaneous, sudden thought, swag, sweep,
   turn, turned around, unadvised, uncalculated, unconditioned reflex,
   unconscious, unconsidered, undeliberate, undeliberated, undesigned,
   unintended, unintentional, unmeditated, unpremeditated, unstudied,
   unthinking, unthinking response, unwilled, unwilling, unwitting,
   urge, vault, wind, wrong-way, wrong-way around