The Collaborative International Dictionary of English v.0.48:

exit poll \ex"it poll`\
   a survey poll taken by interviewing voters as they leave
   (exit) the polling place, to determine how they voted and for
   what reasons; -- it is usually taken by news media to learn
   at an early time (often before the balloting has finished)
   who the winners are, and sometimes the reasons for the
   voters' choices.
   [PJC]

WordNet (r) 3.0 (2006):

exit poll
    n 1: a poll of voters as they leave the voting place; usually
         taken by news media in order to predict the outcome of an
         election