The Collaborative International Dictionary of English v.0.48:

Conestoga wagon \Con`es*to"ga wag`on\ or Conestoga wain
\Con`es*to"ga wain\ [From Conestoga, Pennsylvania.]
   A kind of large broad-wheeled wagon, usually covered, for
   traveling in soft soil and on prairies.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Conestoga wagon
    n 1: a large wagon with broad wheels and an arched canvas top;
         used by the United States pioneers to cross the prairies in
         the 19th century [syn: covered wagon, Conestoga wagon,
         Conestoga, prairie wagon, prairie schooner]