so subtle a sensory stimulus that one is not aware of its existence but may in some manner respond to it
Subliminal influences or messages affect your mind without you being aware of it. Colour has a profound, though often subliminal influence on our senses and moods. subliminal advertising. affecting your mind in a way that you are not conscious of (limen; SUBLIME)
{s} of or pertaining to that which exists or operates below the threshold of consciousness