The Collaborative International Dictionary of English v.0.48:

Hotbed \Hot"bed`\, n.
   1. (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.
      [1913 Webster]

   2. A place which favors rapid growth or development; as, a
      hotbed of sedition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hotbed
    n 1: a situation that is ideal for rapid development (especially
         of something bad); "it was a hotbed of vice"
    2: a bed of earth covered with glass and heated by rotting
       manure to promote the growth of plants

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "hotbed":
   Hydra, birthplace, breeding place, brooder, cloche, cold frame,
   conservatory, coolhouse, cradle, forcing, forcing bed,
   forcing house, forcing pit, glasshouse, greenhouse, hatchery,
   hothouse, incubator, lathhouse, milk cow, mustard, nest, nidus,
   nursery, orangery, pinery, rabbit, rich soil, rookery, seed plot,
   seedbed, summerhouse, warren