The Full form of GMT is Greenwich Mean Time. GMT is the mean solar time at the royal observatory in Greenwich London. It is measured on the zero degree line of longitude of the Earth (meridian). This line runs from the North Pole to South Pole, i.e. passes through the Old Royal Observatory, Greenwich in London.
Greenwich Mean Time (GMT) is considered as a global time standard. Established in the year of 1884, it is used as a benchmark for all time zones and as a standard to scale the time in the world. Because it is measured from the Greenwich Meridian Line, so sometimes, Greenwich Mean Time is also known as Greenwich Meridian Time. It is based on the rotation of the earth which is irregular but it follows a fictitious mean and considers a uniform speed along the equator.
GMT is international time and the basis of world time. This is an absolute time reference and hence, doesn’t change with the season. For example, if a country like China which is located to the east of the Greenwich Meridian, its local time is ahead of GMT, e.g. the local time of China is GMT+8 hours. The longitude of the GMT is 0º 0′ 0″ and the Latitude of GMT is 51º 28′ 38″N (North of the Equator).