A blue plaque is a permanent sign installed in a public place in the United Kingdom, and certain other countries and territories, 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 senses. It may be used narrowly and specifically to refer to the "official" scheme administered by English Heritage, and 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 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.

A commemorative plaque, or simply plaque, or in other places referred to as a historical marker, historic marker, or historic plaque, is a plate of metal, ceramic, stone, wood, or other material, typically attached to a wall, stone, or other vertical surface, and bearing text or an image in relief, or both, to commemorate one or more persons, an event, a former use of the place, or some other thing. Many modern plaques and markers are used to associate the location where the plaque or marker is installed with the person, event, or item commemorated as a place worthy of visit. A monumental plaque or tablet commemorating a deceased person or persons, can be a simple form of church monument. Most modern plaques affixed in this way are commemorative of something, but this is not always the case, and there are purely religious plaques, or those signifying ownership or affiliation of some sort. A plaquette is a small plaque, but in English, unlike many European languages, the term is not typically used for outdoor plaques fixed to walls.