The term 'blazer' originated in 1825 and then meant only 'the red flannel boating jacket worn by the Lady Margaret, St. John's College, Cambridge, Boat Club'.
Nowadays, a blazer is a type of jacket resembling a suit coat cut more casually, typically with metal buttons. It is intended as an outdoor jacket and always solid-colored - usually blue, black, camel or dark green. It is single- or double-breasted and made of sturdy material.