Koi is literally named as "Brocaded Carp", are colored varieties of the Amur carp (Cyprinus rubrofuscus) that are kept for decorative purposes in outdoor koi ponds or water gardens.

Koi is an informal group name of the colored variants of C. rubrofuscus. Several varieties are recognized by the Japanese. Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, orange, yellow, blue, and cream. The most popular category of Koi is the Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.

The hobby of keeping Koi eventually spread worldwide. They are sold in many pet aquarium shops, with higher-quality fish available from specialist dealers.Collecting Koi has become a social hobby. Passionate hobbyists join clubs, share their knowledge and help each other with their Koi.