Scotland Yard (officially New Scotland Yard, though an Old Scotland Yard has never existed) is a metonym for the headquarters of the Metropolitan Police Service, the territorial police force responsible for policing most of London.
The name derives from the location of the original Metropolitan Police headquarters at 4 Whitehall Place, which had a rear entrance on a street called Great Scotland Yard Situated opposite St James Street Station the present location of the headquarters of the Metropolitan Police, where they moved in 1890.
More information on the Wikipedia page [1] and the official website is [2].