A broch is an Iron Age drystone hollow-walled structure of a type found only in Scotland. Brochs include some of the most sophisticated examples of drystone architecture ever created, and belong to the classification "complex Atlantic Roundhouse" devised by Scottish archaeologists in the 1980s. The theory that they were defensive military structures is not accepted by many modern archaeologists, while the alternative notion that they were farmhouses is dismissed by some others. Although most stand alone in the landscape, some examples exist of brochs surrounded by clusters of smaller dwellings. There is controversy about whether or not brochs were roofed.
The word broch is derived from Lowland Scots 'brough', meaning (among other things) fort. In the mid-19th century Scottish antiquaries called brochs 'burgs', after Old Norse borg, with the same meaning. Place names in Scandinavian Scotland such as Burgawater and Burgan show that O.N. borg is the older word used for these structures in the north. Brochs are often referred to as 'duns' in the west. Antiquaries began to use the spelling 'broch' in the 1870s. A precise definition for the word has proved elusive. Brochs are the most spectacular of a complex class of roundhouse buildings found throughout "Atlantic Scotland". The Shetland Amenity Trust lists about 120 sites in Shetland as candidate brochs, while The Royal Commission on the Ancient and Historic Monuments of Scotland identifies a total of 571 candidate broch sites throughout the country.