The Collaborative International Dictionary of English v.0.48:

Mycelium \My*ce"li*um\, n. [NL., fr. Gr. my`khs a mushroom.]
   (Bot.)
   The white threads or filamentous growth from which a mushroom
   or fungus is developed; the so-called mushroom spawn. --
   My*ce"li*al, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mycelium
    n 1: the vegetative part of a fungus consisting of a mass of
         branching threadlike hyphae