mush·room 
(mŭshrm′, -rm′)
(mŭshrm′, -rm′)n.
1.
a. Any of various fungi that produce a fleshy fruiting body, especially one consisting of a stalk with an umbrella-shaped cap.
b. Any of such fungi that are edible, especially the widely cultivated species Agaricus bisporus, which includes the button, cremini, and portobello mushrooms.
c. The usually aboveground fruiting body of any of such fungi.
d. One of these fruiting bodies that produce hallucinations when ingested. Also called magic mushroom.
2. Something shaped like one of these fungi.
intr.v. mush·roomed, mush·room·ing, mush·rooms
1. To multiply, grow, or expand rapidly: The population mushroomed in the postwar decades.
2. To swell or spread out into a shape similar to a mushroom.
3. To collect wild mushrooms.
adj.
1. Relating to, consisting of, or containing mushrooms: mushroom sauce.
2. Resembling mushrooms in rapidity of growth or evanescence: mushroom towns.
[Alteration (influenced by ROOM) of Middle English musheron, from Anglo-Norman moscheron, musherum, from Old French mousseron, from Medieval Latin musariō, musariōn-.]