Mycelium

Mycelium is the vegetative part of a fungus consisting of a mass of branching threadlike hyphae that exists below the ground or within another substrate. It is through the mycelium that a fungus absorbs nutrients from its environment. Mycelium is also a vital component in many ecosystems in that it helps increase the efficiency of water and nutrient absorption of many plants and also is vital to the decomposition and breaking-up of plant material to form the organic part of soil and to release carbon dioxide back into the atmosphere.

The familiar hat-like head and stalk of mushrooms are in fact reproductive structures of ascomycetes and basidiomycetes fungi, and are not classified as mycelium.