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]