The Collaborative International Dictionary of English v.0.48:

brainwave \brain"wave`\ (br[=a]n"w[=a]v), n.
   1. rapid fluctuations of voltage between parts of the brain.

   Syn: brain wave, cortical potential.
        [WordNet 1.5]

   2. the clear (and often sudden) understanding of a complex
      situation.

   Syn: insight, brainstorm.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

brainwave
    n 1: (neurophysiology) rapid fluctuations of voltage between
         parts of the cerebral cortex that are detectable with an
         electroencephalograph [syn: brainwave, brain wave,
         cortical potential]
    2: the clear (and often sudden) understanding of a complex
       situation [syn: insight, brainstorm, brainwave]