A blazer is a distinct type of lightweight sports jacket, traditionally made of navy or striped wool or linen. It is typically characterised by metal buttons on the front and sleeves. In terms of formality within Western dress codes, a blazer has a dressier appearance than other sports jackets, yet is seen as less formal than a suit jacket.

First introduced in the 1860s as a scarlet jacket in club or plain colours for boating or cricket, it began to shed its sporting associations in the 1930s. The double-breasted navy version in particular established itself as a staple of classic style, occupying a space between the casual tweed jacket and the formal dark suit, and soon became linked to the lifestyle of wealthy elites. Coloured variants, however, continue to appear in some circles as semi-active sportswear, notably after cricket or rowing.

A "nautical blazer" is defined as a double-breasted navy jacket with naval-style metal buttons, traditionally gold and sometimes silver. "Rowing" or "cricket blazers" are characterised by brightly coloured stripes and a breast-pocket badge denoting club or college affiliation. Other variations, often with a contrasting piping along notched lapels, are worn in more formal settings such as the presentation of cups or medals.

Blazers are also commonly worn as part of uniforms, for instance by airline employees, school pupils, sports clubs, and competitive teams.

Blazers are worn with a wide variety of clothes, ranging from a dress shirt and necktie to an open-necked polo shirt, or even just a plain t-shirt. They are seen with trousers of all colours and fabrics, from the classic white cotton or linen, to grey flannel, to brown or beige chinos, and also jeans.

A fitted, classically cut, double-breasted navy blue blazer with navy-style buttons is a popular design and sometimes referred to as a "reefer" blazer. Particularly in North America and the UK, it is often used in business casual attire.

Blazers, in a wide range of colours, are worn as part of school uniforms by many schools across the Commonwealth, and are still daily wear for most uniformed pupils in Britain, Ireland, Australia, New Zealand and South Africa. These are blazers in the traditional sense: single-breasted, and often of bright colours or with piping. This style is also worn by some boat clubs, such as those in Cambridge or Oxford, with the piped version used only on special occasions such as a boat club dinner. In this case, the piping is in college colours, and college buttons are worn. This traditional style can be seen in many feature films set in the Edwardian era, such as Kind Hearts and Coronets.

Where the blazer is part of the dress of a school, college, sports club, or armed service regimental association (veterans' organization), it is normal for a badge to be sewn to the breast pocket. In schools, this badge may vary according to the pupil's standing in the school: being a member of the junior or senior school, being a prefect, or having been awarded colours in recognition of particular achievement in some academic or sporting field. In the Commonwealth, many regimental associations wear "regimental blazers" which also sport a similar badge on the breast pocket, usually in the form of a wire badge, and sometimes also regimental blazer buttons.