Seal: Used to describe Boston Terriers, this color appears black except that it has a red cast when viewed in the sun or bright light. Color definitions may vary by breed. Always check the breed standard for the definitive color description.

Season: The cyclical period in which a female dog becomes interested in mating and capable of becoming pregnant.

Secondary Coat: The fine hairs which make up the undercoat.

Second Thigh: That part of the hindquarters from the stifle to the hock, corresponding to the human shin and calf. Lower thigh, including the tibia and fibula.

Sedge: Used to describe Chesapeake Bay Retrievers, this color is similar to deadgrass, but it is more accurately a lightening of the chocolate hue. Color definitions may vary by breed. Always check the breed standard for the definitive color description.

Seeing Eye Dog: A dog trained as a guide dog for the blind.

Selective Breeding: Intentional mating two dogs in order to achieve or eliminate a specific trait.

Self Color: One color or whole color except for lighter shadings. Color definitions may vary by breed. Always check the breed standard for the definitive color description.

Semi-Prick Ears: Ears carried erect with just the tips leaning forward.

Senile Cataracts: Occurring in elderly animals, a lens opacity in the eye that does not interfere with vision.

Septum: The line extending vertically between the nostrils.

Service Dog: A dog trained to assist a person with a disability.

Setters: Any of several breeds of longhaired hunting dogs originally trained to indicate the presence of game by crouching in a set position.