hotbed

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

• noun An environment conducive to vigorous growth or development, especially of something undesirable.
• noun A glass-covered bed of soil heated with fermenting manure or by electricity, used for germinating seeds or protecting tender plants.

from The Century Dictionary.

• noun In horticulture, a bed of earth heated by fermenting substances, and covered with glass to defend it from the cold air, intended for raising early plants, or for protecting tender exotics.
• noun Figuratively, a seat of rapid growth or development, or of eager activity of some kind: generally in a bad sense: as, a hotbed of sedition.
• noun In rail-making, the bed on which the redhot rail taken from the rolls is placed to cool.

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

• noun (Gardening) A bed of earth heated by fermenting manure or other substances, and covered with glass, intended for raising early plants, or for nourishing exotics.
• noun A place which favors rapid growth or development.

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

• noun a low bed of earth covered with glass, and heated with rotting manure; used for the germination of seeds and the growth of tender plants. A hotbed works as a miniature hothouse, i.e. greenhouse.
• noun by extension an environment that is ideal for the growth or development of something, especially of something undesirable

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

• noun a bed of earth covered with glass and heated by rotting manure to promote the growth of plants
• noun a situation that is ideal for rapid development (especially of something bad)