Definition of mushroom:
(n) :
any of the fleshy
fruiting bodies of
fungi typically produced above ground on soil or on their food sources (such as decaying wood); a fungus producing such fruiting bodies
(adj) :
containing or being made of mushrooms; resembling a
mushroom by shape or appearance
"mushroom soup"
"the town's poplulation mushroomed from 10,000 to 110,000 in five years."