A blazer is a type of single breasted coat, closely related to a suit jacket. Generally, it differs from a suit jacket in that the buttons are usually metallic, and the outer material generally more durable. They occur most often in blue colors, but blazers of other colors are not unheard of. They are included often in uniforms of civilian bodies, such as airlines, boys schools, yacht clubs, and private security organizations.

Looking at the classic blazer, the double-breasted blue one with brass buttons, the conclusion that the jacket is of military tradition is rather obvious. Most say the name stems from the HMS Blazer, a frigate, whose captain, when faced with a visit to his ship by Queen Victoria, decided to outfit his crew in short double breasted jackets in navy blue serge, with brass Royal Navy buttons. It is said to have been modeled after the reefer jacket of midshipmen. Apparently the Queen was so smitten with the "uniform" that it became the style. Today, the blazer remains navy blue with brass buttons, although variations can be had in black, tartan green or even red. Colored and striped blazers have been the style since the public school days, and there is also a tale that the red summer jackets of the oarsmen of the Lady Margaret Boat Club, in Cambridge, were said to look "ablaze" and began a tradition which today is seen in single breasted blazers at Henley Regatta.

Arguably, the name could also derive from the traditional practice of embroidering or attaching a school's, regiment's, club's, or ship's coat-of-arms to the breast of the coat - thus emblazoning the garment.