The Collaborative International Dictionary of English v.0.48:

Tiers 'etat \Tiers` ['e]`tat"\ [F.]
   The third estate, or commonalty, in France, answering to the
   commons in Great Britain; -- so called in distinction from,
   and as inferior to, the nobles and clergy.
   [1913 Webster]

   Note: The refusal of the clergy and nobility to give the
         tiers ['e]tat a representation in the States-general
         proportioned to their actual numbers had an important
         influence in bringing on the French Revolution of 1789.
         Since that time the term has been purely historical.
         [1913 Webster]