The Spencer coat is a style of coat dating back to 1790, and was created to be an additional short coat worn over a more full-length coat to add extra protection and warmth across the torso.
The original incarnation of the Spencer coat was nothing more than a double-breasted tailcoat with their tails cut off. Traditionally it was worn over a full long tailed coat. While the Spencer coat started as a mens coat it was quickly adopted by women and worn in the same fashion as the cardigan is worn today.
Today the term Spencer coat has become a generic phrase used to describe any short jacket or coat.