The Collaborative International Dictionary of English v.0.48:

Sentience \Sen"ti*ence\, Sentiency \Sen"ti*en*cy\, n. [See
   Sentient, Sentence.]
   The quality or state of being sentient; esp., the quality or
   state of having sensation. --G. H. Lewes.
   [1913 Webster]

         An example of harmonious action between the
         intelligence and the sentiency of the mind. --Earle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sentience
    n 1: state of elementary or undifferentiated consciousness; "the
         crash intruded on his awareness" [syn: awareness,
         sentience]
    2: the faculty through which the external world is apprehended;
       "in the dark he had to depend on touch and on his senses of
       smell and hearing" [syn: sense, sensation, sentience,
       sentiency, sensory faculty]
    3: the readiness to perceive sensations; elementary or
       undifferentiated consciousness; "gave sentience to slugs and
       newts"- Richard Eberhart [ant: insentience]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "sentience":
   affectibility, alertness, all-night vigil, consciousness,
   impressibility, impressionability, insomnia, insomniac,
   insomnolence, insomnolency, lidless vigil, limen,
   openness to sensation, perceptibility, physical sensibility,
   readiness of feeling, receptiveness, receptivity, restlessness,
   sensation level, sensibility, sensibleness, sentiency,
   sleeplessness, susceptibility, susceptivity,
   threshold of sensation, tossing and turning, vigil, wake,
   wakefulness