Cathay is the Anglicized version of "Catai" and an alternative name for China in English.
It originates from the word Khitan (Chinese: 契丹, Qìdān), the name of a nomadic people who founded the Liao Dynasty which ruled much of Northern China from 907 to 1125, and who had a state of their own (Kara-Khitan Khanate) centered around today's Kyrgyzstan for another century thereafter.
Originally, Catai was the name applied by Central and Western Asians and Europeans to northern China; it obtained wide currency in Europe after the publication of Marco Polo's book (he referred to southern China as Manji).
For centuries Cathay and China were believed by Europeans to be distinct nations with distinct cultures. However, by the late 1600s Europeans had mostly become aware that these were in fact the same nation.