1. flattened disk-shaped stony coral (usually solitary and unattached)
The Collaborative International Dictionary of English v.0.48:
Mushroom \Mush"room\, a.
1. Of or pertaining to mushrooms; as, mushroom catchup.
2. Resembling mushrooms in rapidity of growth and shortness
of duration; short-lived; ephemerial; as, mushroom cities.
Mushroom anchor, an anchor shaped like a mushroom, capable
of grasping the ground in whatever way it falls.
Mushroom coral (Zool.), any coral of the genus Fungia.
Mushroom spawn (Bot.), the mycelium, or primary filamentous
growth, of the mushroom; also, cakes of earth and manure
containing this growth, which are used for propagation of
mushroom cloud, a cloud of smoke rising and then spreading
laterally to take on the shape of a mushroom -- caused by
large fires or explosions, esp. nuclear explosions.
[1913 Webster +PJC]