Wikipedia has a detailed definition of fungus that begins as follows:
"A fungus (plural: fungi[2] or funguses[3]) any member of the group of eukaryotic organisms that
includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms
are classified as a kingdom, which is separate from the other eukaryotic life kingdoms of plants and animals.
A characteristic that places fungi in a different kingdom from plants, bacteria,
and some protists is chitin in their cell walls. Similar to animals, fungi are
heterotrophs; they acquire their food by absorbing dissolved molecules, typically by
secreting digestive enzymes into their environment. Fungi do not photosynthesize.
Growth is their means of mobility, except for spores (a few of which are flagellated),
which may travel through the air or water. Fungi are the principal decomposers in ecological
systems..."