The Collaborative International Dictionary of English v.0.48:

Somnambulism \Som*nam"bu*lism\, n. [Cf. F. somnambulisme. See
   Somnambulation.]
   A condition of the nervous system in which an individual
   during sleep performs actions appropriate to the waking
   state; a state of sleep in which some of the senses and
   voluntary powers are partially awake; noctambulism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

somnambulism
    n 1: walking by a person who is asleep [syn: sleepwalking,
         somnambulism, somnambulation, noctambulism,
         noctambulation]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "somnambulism":
   amnesia, beauty sleep, beddy-bye, bedtime, blanket drill, bye-bye,
   catalepsy, cataplexy, catatonic stupor, daydreaming, daze, doze,
   dream state, dreamland, drowse, fitful sleep, fugue, fugue state,
   hibernation, hypnotic trance, land of Nod, light sleep,
   night-wandering, nightwalking, noctambulation, noctambulism,
   noctivagation, repose, reverie, shut-eye, silken repose, sleep,
   sleepland, sleepwalk, sleepwalking, slumber, slumberland, snoozle,
   somniloquy, somnus, stupor, trance, unconsciousness,
   winter sleep



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SOMNAMBULISM, med. juris. Sleep walking.
     2. This is sometimes an inferior species of insanity, the patient being
unconscious of what he is doing. A case is mentioned of a monk who was
remarkable for simplicity, candor and probity, while awake, but who during
his sleep in the night, would steal, rob, and even plunder the dead. Another
case is related of a pious clergyman, who during his sleep, would plunder
even his own church. And a case occurred in Maine, where the somnambulist
attempted to hang himself, but fortunately tied the rope to his feet,
instead of his neck. Ray. Med. Jur. Sec. 294.
     3. It is evident, that if an act should be done by a sleep walker,
while totally unconscious of his act, he would not be liable to punishment,
because the intention (q.v.) and will (q.v.) would be wanting. Take, for
example, the following singular case: A monk late one evening, in the
presence of the prior of the convent, while in a state of somnambulism,
entered the room of the prior, his eyes open but fixed, his features
contracted into a frown, and with a knife in his hand. He walked straight up
to the bed, as if to ascertain if the prior were there, and then gave three
stabs, which penetrated the bed clothes, and a mat which served for the
purpose of a mattress; he returned. with an air of satisfaction, and his
features relaxed. On being questioned the next day by the prior as to what
he had dreamed the preceding night, the monk confessed he had dreamed that
his mother had been murdered by the prior, and that her spirit had appeared
to him and cried for vengeance, that he was transported with fury at the
sight, and ran directly to stab the assassin; that shortly after be awoke
covered with perspiration, and rejoiced to find it was only a dream.
Georget, Des Maladies Mentales, 127.
     4. A similar case occurred in England, in the last century. Two
persons, who had been hunting in the day, slept together at night; one of
them was renewing the chase in his dream, and, imagining himself present at
the death of the stag, cried out aloud, "I'll kill him! I'll kill him!" The
other, awakened by the noise, got out of bed, and, by the light of the moon,
saw the sleeper give several deadly stabs, with a knife, on the part of the
bed his companion had just quitted. Harvey's Meditations on the Night, note
35; Guy, Med. Jur. 265.