Greenwich Mean Time (GMT) is a time system originally referring to mean solar time at the Royal Observatory in Greenwich, London, which later became adopted as a global time standard. It is arguably the same as Coordinated Universal Time (UTC) and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others.

Before the introduction of UTC on 1 January 1972 Greenwich Mean Time (also known as Zulu time) was the same as Universal Time (UT) which is a standard astronomical concept used in many technical fields. Astronomers no longer use the term "Greenwich Mean Time".

In the United Kingdom, GMT is the official time only during winter; during summer British Summer Time is used. GMT is the same as Western European Time.

Noon Greenwich Mean Time is rarely the exact moment when the sun crosses the Greenwich meridian (and reaches its highest point in the sky at Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (a discrepancy calculated by the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

Coordinates: 51°28′45″N 0°00′00″E / 51.4791°N 0.0000°E / 51.4791; 0.0000

Greenwich (UK ⁱ/ɡrɪnɪdʒ/ GRIN-ij; US /ɡrɛnɪtʃ/ GREN-ich or /ɡrɛnɪdʒ/ GREN-ij) is a district of south London, England, located in the Royal Borough of Greenwich and situated 5.5 miles (8.9 km) east south-east of Charing Cross.

Greenwich is notable for its maritime history and for giving its name to the Greenwich Meridian (0° longitude) and Greenwich Mean Time. The town became the site of a royal palace, the Palace of Placentia from the 15th century, and was the birthplace of many in the House of Tudor, including Henry VIII and Elizabeth I. The palace fell into disrepair during the English Civil War and was rebuilt as the Royal Naval Hospital for Sailors by Sir Christopher Wren and his assistant Nicholas Hawksmoor. These buildings became the Royal Naval College in 1873, and they remained an establishment for military education until 1998 when they passed into the hands of the Greenwich Foundation. The historic rooms within these buildings remain open to the public; other buildings are used by University of Greenwich and the Trinity College of Music.