Noun
A solid mass of cellular tissue, consisting of one or more layers, usually in the form of a flat stratum or expansion, but sometimes erect or pendulous, and elongated and branching, and forming the substance of the thallogens.
Source: Webster's dictionaryBy extension, the term "mushroom" can also designate the entire fungus when in culture; the thallus (called a mycelium ) of species forming the fruiting bodies called mushrooms; or the species itself. Source: Internet
Some are self-compatible and spontaneously form dikaryons without a separate compatible thallus being involved. Source: Internet