The Collaborative International Dictionary of English v.0.48:

Incubator \In"cu*ba`tor\, n.
   1. That which incubates, especially, an apparatus by means of
      which eggs are hatched by artificial heat.
      [1913 Webster]

   2. An apparatus containing an enclosed chamber, used for the
      cultivation of micro["o]rganisms or tissue cultures by
      maintaining a suitable temperature and atmospheric
      composition. Some incubators have no provision for
      maintaining a special atmosphere, while in others,
      especially for anaerobic organisms and tissue culture, the
      moisture level and composition of the gases are also
      controlled.
      [Webster 1913 Suppl. +PJC]

   3. (Med.) An apparatus consisting of enclosed chamber, for
      maintaining prematurely born babies in a favorable
      environment until able to thrive under normal conditions.
      The temperature and level of oxygen in the atmosphere may
      be controlled.
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

incubator
    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]