The Collaborative International Dictionary of English v.0.48:

brownout \brown"out\ n.
   1. darkness resulting from the extinction of lights (as in a
      city invisible to enemy aircraft).

   Syn: blackout, dimout.
        [WordNet 1.5]

   2. a partial reduction in the amount of electric power
      available to customers in a region, such as by reduction
      of voltage or selective cutoff of certain customers; -- it
      occurs for example in summer when the demand for
      electricity for air-conditioning exceeds the supply, or
      when power to certain customers is cut off by a severe
      storm. It is contrasted with blackout, which is the
      total loss of electrical power in a region.
      [PJC]

WordNet (r) 3.0 (2006):

brownout
    n 1: darkness resulting from the extinction of lights (as in a
         city invisible to enemy aircraft) [syn: blackout,
         brownout, dimout]