London Derriere is a term used to describe the London skyline, which is characterized by its iconic buildings and monuments. The term is derived from the French phrase 'derrière le dos de Londres', which translates to 'behind the back of London'. The crossword solution for this term is BUM, which stands for Built Up Metropolis. This term is used to describe the densely populated urban areas of London, which are home to millions of people.