1. 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.
2. An intensely hot place: the furnace of the sun; an attic room that is a furnace in the summer.
3. A severe test or trial: endured the furnace of his friends' blame after the accident.

[Middle English, from Old French fornais, from Latin fornāx, fornāc-, oven.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.