An inclosed place in which heat is produced by the combustion of fuel, as for reducing ores or melting metals, for warming a house, for baking pottery, etc.; as, an iron furnace; a hot-air furnace; a glass furnace; a boiler furnace, etc.
- An enclosure in which energy in a nonthermal form is converted to heat, especially such an enclosure in which heat is generated by the combustion of a suitable fuel.
- An intensely hot place: the furnace of the sun; an attic room that is a furnace in the summer.
- A severe test or trial: endured the furnace of his friends' blame after the accident.