Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

subliminal
     adj : below the threshold of conscious perception