A blue plaque is a permanent sign installed in a public place in the United Kingdom and elsewhere to commemorate a link between that location and a famous person, event, or former building on the site, serving as a historical marker. The term is used in the United Kingdom in two different senses. It may be used narrowly and specifically to refer to the "official" scheme administered by English Heritage, and currently restricted to sites within Greater London; or it may be used less formally to encompass a number of similar schemes administered by organisations throughout the UK.
The "official" scheme traces its origins to that launched in 1866 in London, on the initiative of the politician William Ewart, to mark the homes and workplaces of famous people. It has been administered successively by the Society of Arts (1866–1901), the London County Council (1901–1965), the Greater London Council (1965–1986) and English Heritage (1986 to date). It remains focused on London (now defined as Greater London), although between 1998 and 2005, under a trial programme since discontinued, 34 plaques were erected elsewhere in England. The first such scheme in the world, it has directly or indirectly provided the inspiration and model for many others.
Many other plaque schemes have since been initiated in the United Kingdom. Some are restricted to a specific geographical area, others to a particular theme of historical commemoration. They are administered by a range of bodies including local authorities, civic societies, residents' associations and other organisations such as the Transport Trust, the Royal Society of Chemistry, the Music Hall Guild of Great Britain and America, the Centre For Pagan Studies and the British Comic Society. The plaques erected are made in a variety of designs, shapes, materials and colours: some are blue, others are not. However, the term "blue plaque" is often used informally to encompass all such schemes.
There are also commemorative plaque schemes throughout the world such as those in Paris, Rome, Oslo, Dublin; and in other cities in Australia, Canada, the Philippines, Russia and the United States. These take various forms, and they are more likely to be known as commemorative plaques or historical markers.