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.
Cathay is often translated nowadays as 国泰 (in simplified Chinese characters) and 國泰 (in traditional Chinese characters), such as in Cathay Pacific Airways (國泰航空公司).
Read more about Cathay: History, Cathay and Mangi, Identifying China As Cathay, Etymological Progression, Use in English, In The Names of Institutions, References in Popular Culture, Literature