A "kyu" is a student rank in Japanese traditions. These later were famously introduced into Judo and other modern martial arts along with the practice of colored belts to represent the different "kyu" ranks. A student starts with a higher-number "kyu" rank, signified in Judo with a white belt, and proceeds to count down lower-number ranks as they improve. After a student attains sufficient "kyu" ranking, they move over to a "dan" ranking which is signified in Judo by a black belt, and instead starting from a lower-number and counting up as one improves in "dan" rank.

While it was introduced to martial arts by Judo, it is not specifies to martial arts and exists in other traditional Japanese practices such as calligraphy

‍