A tonne (also called metric ton) is a non-SI unit of mass, accepted for use with SI, defined as:
It is approximately equal to 2,204.62262 pounds.
The tonne spelling is borrowed from the French word, and is now common in English where it is rarely used for any other ton. For the United States, metric ton is the name for this unit used and recommended by NIST.  http://physics.nist.gov/Pubs/SP811/sec05.html#5.1.1
The official symbol is t, but T and mT and mt (especially in the combination mmt for "million metric tons") are also used.
The tonne may also be referred to by the strictly SI term megagram, symbol Mg.
Like grams and kilograms, tonnes have also given rise to a force unit of the same name: 1 tonne-force = 9.80665 kilonewtons (kN), a unit also often called simply "tonne" or "metric ton" without identifying it as a unit of force. Note that it is only the tonne as a unit of mass which is accepted for use with SI; the tonne-force or metric ton-force is not acceptable for use with SI.
The ton of TNT or tonne of TNT is a unit of energy based on the tonne, assuming 1000 small calories per gram (4.184 kJ/g) and thus a tonne of TNT is 4.184 GJ. This unit is also not acceptable for use with SI.
kilogram « tonne/megagram « gigagram
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI) http://physics.nist.gov/Pubs/SP811/