n. 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.n. 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.n. In rail-making, the bed on which the redhot rail taken from the rolls is placed to cool.