Teri

Skin shine. Koi produce a secretion called the slime coat on the surface of the body to protect the skin. Healthy Koi produce a lot of this secretion and it creates a shine over the body. Unhealthy Koi produce less secretion and thus the skin does not seem to have sheen. This sheen is called Teri.

(Courtesy of the excellent Book Kokugyo Vol II)