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.
      [1913 Webster]

            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]

WordNet (r) 3.0 (2006):

patellar reflex
    n 1: a reflex extension of the leg resulting from a sharp tap on
         the patellar tendon [syn: knee jerk, knee-jerk reflex,
         patellar reflex]