Cathay is a really old word that used to be used a long, long time ago to talk about places far away. Think about it like a fairy tale land or a magical kingdom that you might hear about in stories!
People used to use the word Cathay to talk about a part of the world that is now called China. Back then, people didn't know as much about China as they do now, so it was kind of a mystery to them. Cathay was like a far-off land filled with exotic animals, spices, and treasures.
Nowadays, you might hear the word Cathay used to talk about a famous airline called Cathay Pacific, which started in Hong Kong, which is in China. So, even though the word Cathay is very old, it still pops up in different ways today!