The O2 is a large entertainment district on the Greenwich peninsula in South East London, England, including an indoor arena, a music club, a Cineworld cinema, an exhibition space, piazzas, bars, restaurants, and a guided tour to the top of the O2. It was built largely within the former Millennium Dome, a large dome-shaped canopy built to house an exhibition celebrating the turn of the third millennium; consequently The Dome remains a name in common usage for the venue. It is sometimes referred to as The O2 Arena, but that name properly refers to the indoor arena within The O2. Naming rights to the district were purchased by the mobile telephone provider O2 from its developers, Anschutz Entertainment Group (AEG), during the development of the district. AEG owns the long-term lease on the O2 Arena and surrounding leisure space.

The Millennium Dome was the original name of the large dome-shaped building on the Greenwich Peninsula in South East London, England, which housed a major exhibition celebrating the beginning of the third millennium. As of 2022, it is the ninth largest building in the world by usable volume. The exhibition was open to the public from 1 January to 31 December 2000. The project and exhibition were highly contentious and attracted barely half of the 12 million customers its sponsors forecasted, and so were deemed a failure by the press. All the original exhibition elements were sold or dismantled.