The Collaborative International Dictionary of English v.0.48:

Domino theory \Dom"i*no the"o*ry\
   A political theory current in the 1960's, according to which
   the conversion of one country in South Asia to communism will
   start a sequential process causing all Asian countries to
   convert to Communism.

   Note: The apparent assumption was that an Asian country
         politically aligned with the West was as politically
         unstable as a domino standing on edge. It was used by
         some as a justification for American involvement in the
         Vietnam war, 1964-1972.
         [PJC]

WordNet (r) 3.0 (2006):

domino theory
    n 1: the political theory that if one nation comes under
         communist control then neighboring nations will also come
         under communist control