The Collaborative International Dictionary of English v.0.48:

slime mold \slime mold\, slime mould \slime mould\n.
   1. An unusual fungus-like protist of the phylum Myxomycota or
      the class Myxomycetes, having a stage of growth in which
      it comprises a naked noncellular multinucleate mass of
      creeping protoplasm having characteristics of both plants
      and animals; it also has a propagative phase in which it
      develops fruiting bodies bearing spores; it is sometimes
      classified as a protist. It is called also acellular
      slime mold. [WordNet 1.5 +PJC]

   2. Any of several remarkable amoebalike organisms of the
      phylum Acrasiomycota, mostly terrestrial, having a
      fruiting phase resembling that of the acellular slime
      molds, but being cellular and nucleate throughout their
      life cycle; called also cellular slime mold. The most
      studied species is Dictyostelium discoideum. In their
      feeding phase, they live like amoebae as individual cells,
      engulfing bacteria as a prime food source. When the food
      source diminishes, they begin to aggregate, swarming
      together to form clumps which may move toward heat and
      light, so as to reach the surface of the ground; they then
      differentiate into a form with spores contained within a
      sporangium resting on a stalk. When the spores are carried
      to another location with adequate food supplies, the
      spores may germinate to resume the life cycle. The phase
      of aggregation appears to be initiated by release of
      cyclic AMP, serving as a signal between the individual
      cells. The formation of the fruiting body has some
      similarities to differentiation in multicellular
      organisms, but the mechanisms are still under study. Some
      biologists object to the classification of Dictyostelium
      as a slime mold, as it is neither a mold nor slimy.
      [PJC]

WordNet (r) 3.0 (2006):

acellular slime mold
    n 1: a slime mold of the class Myxomycetes [syn: true slime
         mold, acellular slime mold, plasmodial slime mold,
         myxomycete]