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]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "reflex action":
   action and reaction, answer, automatic reaction, automatic writing,
   automatism, autonomic reaction, blind impulse, bounceback,
   compulsiveness, conditioning, echo, echolalia, echopraxia, impulse,
   instinct, instinctiveness, involuntariness, predictable response,
   reaction, reflection, reflex, refluence, reflux, reply,
   respondence, response, retroaction, return, reverberation,
   revulsion, rise, sheer chemistry, unthinking response,
   unwilledness