sclerotium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hard, dormant, multicellular structure found in certain fungi and slime molds.

from The Century Dictionary.

• noun In botany:
• noun A pluricellular tuber-like reservoir of reserve material forming on a primary filamentous mycelium, from which it becomes detached when its development is complete.
• noun [capitalized] An old genus of fungi, comprising hard, black, compact bodies which are now known to be a resting-stage of the mycelium of certain other fungi, such as Peziza tuberosa. See ergot, 2.
• noun In zoology, one of the peculiar quiescent cysts or hypnocysts of Mycetozoa, not giving rise to spores.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A hardened body formed by certain fungi, as by the Claviceps purpurea, which produces ergot.
• noun (Zoöl.) The mature or resting stage of a plasmodium.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A compact mass of hardened mycelium stored with reserve food material that, in some higher fungi such as ergot, becomes detached and remains dormant until a favourable opportunity for growth occurs.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun compact usually dark-colored mass of hardened mycelium constituting a vegetative food-storage body in various true fungi; detaches when mature and can give rise to new growth
• noun form genus of sterile imperfect fungi; many form sclerotia; some cause sclerotium disease in plants