Greenwich Mean Time (GMT) is the time measured on the Earth's zero degree line of longitude, or meridian.
This runs from the North Pole to the South Pole, passing through the Old Royal Observatory in the London suburb of Greenwich.
This line has been called the Greenwich Meridian since 1884, and it is from here that all terrestrial longitudes are measured and the world's time zones are calculated.
Generally, if you are in a country east of the Greenwich Meridian, your local time is ahead of GMT (e.g. local time in China is GMT +8 hours).
West of the Greenwich Meridian, local time is behind GMT (e.g. local time in New York is GMT -5 hours in winter and GMT -4 hours in summer).
BBC World Service and GMT
BBC World Service times are normally shown in GMT, although our online schedules will change in March to GMT +1 which is in line with British Summer Time (BST).
The dividing line between East (GMT+) and West (GMT-) on the opposite side of the world to the Greenwich Meridian is the International Date Line.
This is a modification of the 180° meridian running north to south through the Pacific Ocean.
GMT remains constant throughout the year.
In the winter months, local time in the UK is the same as GMT, but in March, local time is moved forward one hour to British Summer Time (BST) until the end of October.
A number of other countries around the world also use this daylight savings measure and change their local times to take advantage of earlier sunrises.
Some broadcasters show times in UTC (Co-ordinated Universal Time).
This is essentially the same as GMT, but UTC is measured by an atomic clock and is thus more accurate - by split seconds.
It is used primarily for scientific purposes.