the filamentous vegetative portion of a fungus, specifically excluding the fruiting structure or reproductive phase of the life cycle. The mycelium may be invisible or conspicuous. In some cases mycelial strands may join to form thick strands called rhizomorphs.
(more than one = mycelia) The thread-like body of a fungus. Most people think of mushrooms when they think of fungi, but mushrooms are just a part of the organism. Mushrooms "bloom" much like flowers do at certain times of the year. The actual main part of the fungus is the mycelia.
The portion of the mushroom that grows underground. Plants have roots; mushrooms have mycelium. Mycelium networks can be huge. The largest living thing in the world is a single underground mycelium complex.