by Encyclopaedia Britannica, 11th Edition
CATHAY, the name by which China (q.v.) was known to medieval Europe and is still occasionally referred to in poetry, as in Tennyson’s “Better fifty years of Europe than a cycle of Cathay.” It is derived from Khitai, or Khitat, the name which was properly that of the kingdom established by the Khitan conquerors in the northern provinces of China about a.d. 907, which after the fall of this dynasty in 1125 remained attached to their former territory, and was subsequently applied by the nations of Central Asia to the whole of China. Thus “Kitai” is still the Russian name for China. The name penetrated to Europe in the 13th century with the fame of the conquests of Jenghiz Khan. After the discovery of southern China by European navigators Cathay was erroneously believed to be a country to the north of China, and it was the desire to reach it that sent the English adventurers of the 16th century in search of the north-east passage.