The Spencer was orginally a woollen outer tail coat without the tails and dates to around the 1790's. It was worn as a short waist length, double breasted, man's jacket and originally named for George Spencer, 2nd Earl of Spencer (1758-1834). (A relation of the late Diana Spencer, Princess of Wales).
The design was also popular with women and was worn as a cardigan or as a short fitted jacket cut just above waist level, or, in Empire style, to the bust line, and tailored on identical lines to the dress.
The use of the term Spencer continued well into the 19th century to mean more gernerally any type of short jacket or coat. For instance in Australia the term is sometimes used to refer to thermal underwear.
In menswear a Spencer is often reffered to as a knitted vest or waistcoat.http://uk.ebid.net/for-sale/vintage-sirdar-knitting-pattern-7796-ladys-spencers-tops-or-jumpers-34-48-130975100.htm