The Collaborative International Dictionary of English v.0.48:

Sentient \Sen"ti*ent\, a. [L. sentiens, -entis, p. pr. of
   sentire to discern or perceive by the senses. See Sense.]
   Having a faculty, or faculties, of sensation and perception.
   Specif. (Physiol.), especially sensitive; as, the sentient
   extremities of nerves, which terminate in the various organs
   or tissues.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sentient \Sen"ti*ent\, n.
   One who has the faculty of perception; a sentient being.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sentient
    adj 1: endowed with feeling and unstructured consciousness; "the
           living knew themselves just sentient puppets on God's
           stage"- T.E.Lawrence [syn: sentient, animate] [ant:
           insensate, insentient]
    2: consciously perceiving; "sentient of the intolerable load";
       "a boy so sentient of his surroundings"- W.A.White

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "sentient":
   alive, alive to, au courant, awake, aware, cognizant, conscious,
   conversant, emotionable, feeling, impressible, impressionable,
   impressive, knowing, open, perceptive, reactive, receptive,
   responsive, sensible, sensile, sensitive, sensitive to,
   susceptible, susceptive, witting