A metric ton (t), also referred to as a "tonne," is a unit of mass in the metric system of units. It is not an SI unit. It is one of a number of definitions of the ton that is used. The term "metric ton" is mostly used in the US to differentiate it from the US ton, which has a measure that is slightly larger than the metric ton. Elsewhere, it is usually referred to as the tonne.
All the various definitions of a ton are abbreviated as "t." As such, it is important to know which definition of the ton is being used, or to specify which version is being used so as to not confuse their values in the metric, US customary, or imperial systems of measurement.
Metric tons are used alongside kilograms (kg) in countries that use the metric system. US tons are used in the US customary system of measurement and are mostly used in the United States and Canada. Imperial tons were the definition of the ton used in the United Kingdom before the UK began the process of metrication. In some cases the imperial ton may still be used in the UK, but generally the UK now uses metric measurements. Below are the equivalences of the various definitions of the ton to kilograms and pounds (lb).
1 metric ton = 1000 kg = 2204.622 lb
1 US ton = 907.18474 kg = 2000 lb
1 imperial ton = 1016.0469 kg = 2240 lb
Metric tons are defined based on the kilogram, while US and imperial tons are defined based on the pound, which is why they have round values when defined using units in their respective systems of measurement.