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.
      [1913 Webster]

   2. Produced in reaction, in resistance, or in return.
      [1913 Webster]

   3. (Physiol.) Of, pertaining to, or produced by, stimulus or
      excitation without the necessary intervention of
      consciousness.
      [1913 Webster]

   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 action
    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]