The Collaborative International Dictionary of English v.0.48:

brooder \brood"er\ n.
   a box designed to maintain a constant temperature by the use
   of a thermostat; used for chicks or premature infants.

   Syn: incubator.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

brooder
    n 1: apparatus consisting of a box designed to maintain a
         constant temperature by the use of a thermostat; used for
         chicks or premature infants [syn: incubator, brooder]