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