The Collaborative International Dictionary of English v.0.48:

Subliminal \Sub*lim"i*nal\, a. [Pref. sub- + L. limen
   threshold.]
   1. (Philos.) Existing in the mind, but below the surface or
      threshold of consciousness; that is, existing as feeling
      rather than as clear ideas.
      [Webster 1913 Suppl.]

   2. having or using an intensity of sensory stimulus
      insufficient to be perceived consciously, but having an
      effect on unconscious mental processes; as, subliminal
      advertising.

   Note: The existence of such an effect is still not
         universally accepted.
         [PJC]

WordNet (r) 3.0 (2006):

subliminal
    adj 1: below the threshold of conscious perception