1. Any of a group of unicellular, multicellular, or syncytial spore-producing organisms feeding on organic matter, including molds, yeast, mushrooms, and toadstools.
2. Fungal infection (especially on fish).
3. Used to describe something that has appeared or grown rapidly and is considered unpleasant or unattractive.
Plural noun: Funguses.
There was a fungus of outbuildings behind the house.