The Collaborative International Dictionary of English v.0.48:

Cathay \Ca*thay"\, n.
   China; -- an old name for the Celestial Empire, said have
   been introduced by Marco Polo and to be a corruption of the
   Tartar name for North China (Khitai, the country of the
   Khitans.)
   [1913 Webster]

         Better fifty years of Europe than a cycle of Cathay.
                                                  --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Cathay
    n 1: a communist nation that covers a vast territory in eastern
         Asia; the most populous country in the world [syn: China,
         People's Republic of China, mainland China, Communist
         China, Red China, PRC, Cathay]