Koi are ornamental varieties of domesticated common carp (Cyprinus carpio) that are kept for decorative purposes in outdoor koi ponds or water gardens.
Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, 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 word koi comes from Japanese, simply meaning "carp." It includes both the dull grey fish and the brightly colored varieties. What are known as koi in English are referred to more specifically as nishikigoi in Japan (literally meaning "brocaded carp"). In Japanese, koi is a homophone for another word that means "affection" or "love"; koi are therefore symbols of love and friendship in Japan.
Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, yellow, blue, and cream. While the possible colors are virtually limitless, breeders have identified and named a number of specific categories. The most popular category is Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.
New koi varieties are still being actively developed. Ghost koi developed in the 1980s have become very popular in the United Kingdom; they are a hybrid of wild carp and Ogon koi, and are distinguished by their metallic scales. Butterfly koi (also known as longfin koi, or dragon carp), also developed in the 1980s, are notable for their long and flowing fins. They are hybrids of koi with Asian carp. Butterfly koi and ghost koi are considered by some to be not true nishikigoi.