vegetative body of a fungus. Network of branched. fungal filaments (hyphae).

Merriam-Webster Online Dictionary
mycelium (noun)
the mass of interwoven filamentous hyphae that forms especially the vegetative portion of the thallus of a fungus and is often submerged in another body (as of soil or organic matter or the tissues of a host) , also a similar mass of filaments formed by some bacteria (as streptomyces)
mycelium (Wikipedia)
Fungal mycelium
Microscopic view of a mycelium.  This image covers a one-millimeter square.
Another microscopic view of a mycelium.  Numbered ticks are 230 µm apart.
Oyster mushroom (Pleurotus ostreatus) growing on coffee grounds
Mycelium as seen under a log

Mycelium is the vegetative part of a fungus or fungus-like bacterial colony, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates into a homokaryotic mycelium, which cannot reproduce sexually; when two compatible homokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or it may be extensive, as in Armillaria ostoyae:

Is this the largest organism in the world? This 2,400-acre [970-hectare] site in eastern Oregon had a contiguous growth of mycelium before logging roads cut through it. ... Mushroom-forming forest fungi are unique in that their mycelial mats can achieve such massive proportions.

— Paul Stamets, Mycelium Running

Through the mycelium, a fungus absorbs nutrients from its environment. It does this in a two-stage process. First, the hyphae secrete enzymes onto or into the food source, which break down biological polymers into smaller units such as monomers. These monomers are then absorbed into the mycelium by facilitated diffusion and active transport.

Mycelia are vital in terrestrial and aquatic ecosystems for their role in the decomposition of plant material. They contribute to the organic fraction of soil, and their growth releases carbon dioxide back into the atmosphere (see carbon cycle). Ectomycorrhizal extramatrical mycelium, as well as the mycelium of Arbuscular mycorrhizal fungi increase the efficiency of water and nutrient absorption of most plants and confers resistance to some plant pathogens. Mycelium is an important food source for many soil invertebrates.

"Mycelium", like "fungus", can be considered a mass noun, a word that can be either singular or plural. The term "mycelia", though, like "fungi", is often used as the preferred plural form.

Sclerotia are compact or hard masses of mycelium.

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