Big Ben is the nickname for the Great Bell of the Great Clock of Westminster, and by extension for the clock tower itself, which stands at the north end of the Palace of Westminster in London, England. Originally known simply as the Clock Tower, it was renamed Elizabeth Tower in 2012 to mark the Diamond Jubilee of Elizabeth II. The clock is a striking clock with five bells.

Clock towers are a specific type of structure that house a turret clock and have one or more clock faces on the upper exterior walls. Many clock towers are freestanding structures but they can also adjoin or be located on top of another building. Some other buildings also have clock faces on their exterior but these structures serve other main functions.