The Collaborative International Dictionary of English v.0.48:

Sclerotium \Scle*ro"ti*um\, n.; pl. Sclerotia. [NL., fr. Gr.
   sklhro`s hard.]
   1. (Bot.) A hardened body formed by certain fungi, as by the
      Claviceps purpurea, which produces ergot.
      [1913 Webster]

   2. (Zool.) The mature or resting stage of a plasmodium.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Sclerotium
    n 1: form genus of sterile imperfect fungi; many form sclerotia;
         some cause sclerotium disease in plants [syn: Sclerotium,
         genus Sclerotium]
    2: compact usually dark-colored mass of hardened mycelium
       constituting a vegetative food-storage body in various true
       fungi; detaches when mature and can give rise to new growth