Mycelium (pl.: mycelia) [a] is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Its normal form is that of branched, slender, entangled, anastomosing, hyaline threads. Fungal colonies composed of mycelium are found in and on soil and many other substrates.

The mycelium is the part of a fungus that is usually underground (or inside some another substance). It is composed of hyphae, which look like threads, or rootlets. The mat of hyphae may be very thickly woven. The fungus uses it to extract nutritients. The hyphae are usually syncytic, that is, they do not have complete cell walls.