The Collaborative International Dictionary of English v.0.48:

Syncytium \Syn*cy"ti*um\, n.; pl. Syncitia. [NL., from Gr. ?
   together + ? a hollow vessel.]
   1. (Biol.) Tissue in which the cell or partition walls are
      wholly wanting and the cell bodies fused together, so that
      the tissue consists of a continuous mass of protoplasm in
      which nuclei are imbedded, as in ordinary striped muscle.
      [1913 Webster]

   2. (Zool.) The ectoderm of a sponge.
      [1913 Webster] Syndactyl

WordNet (r) 3.0 (2006):

syncytium
    n 1: a mass of cytoplasm containing several nuclei and enclosed
         in a membrane but no internal cell boundaries (as in muscle
         fibers)