Longman Dictionary of Contemporary English
WordNet
n. lightweight single-breasted jacket; often striped in the colors of a club or school [syn: blazer, sport jacket, sports jacket, sports coat]
Wikipedia
A sport coat ( American English), also called a sports jacket ( British English), less commonly known as a sportcoat, is a jacket for men. Though it is of a similar cut and length to a suit jacket, a sport coat is less formal and is designed to be worn on its own without matching trousers. Styles, fabrics, colours and patterns are more varied than in most suits; sturdier and thicker fabrics are commonly used, such as corduroy, suede, denim, leather, and tweed.
Originally, sports coats were worn as casual attire for watching outdoor sports. With time, they came to be used on more formal occasions, sometimes being used in school uniforms.