A bed of earth heated by fermenting manure or other substances, and covered with glass, intended for raising early plants, or for nourishing exotics.
A hotbed is a cold frame modified to provide additional heat for the plants inside. This can be accomplished by wiring your cold frame to accept a heating element. The element should be imbedded in sand and vermiculite a few inches below plants in containers. More simply, though, a cold frame can be set above a deep bed of horse manure (about 2' deep). The horse manure decomposes slowly over the winter, all the while releasing heat. Again, plants should be in containers to avoid being burned by the salts in the manure.