Mycelium
The vegetative body of a fungus — a network of filaments that lives mostly hidden in substrate.
Mycelium is the vegetative part of a fungus, consisting of a vast network of fine filaments called hyphae. While the mushroom you see is the reproductive structure of the organism, the mycelium is the much larger living body — typically hidden underground, in wood, or within other substrate.
A single mycelial network can extend through cubic meters of soil, with hyphae packed densely enough that a teaspoon of healthy forest topsoil can contain hundreds of meters of hyphal length. The largest documented single organism on Earth is a mycelium of Armillaria ostoyae covering approximately ten square kilometers of forest floor in Oregon.
Mycelia acquire nutrients by secreting extracellular enzymes that break down complex organic molecules, then absorbing the smaller molecules across the hyphal cell wall. Many species form mutualistic relationships with plant roots — mycorrhizal partnerships that exchange water and minerals for plant sugars.