Thermodynamic or Kelvin scale
The thermodynamic scale is the one that is used for scientific measurement. It is measured in units called kelvins (K), the temperature itself being given the letter T. It is defined using one fixed point - the triple point of water. This is the temperature where saturated water vapour, pure water and ice are all in equilibrium at a temperature of 273.16 K.

Absolute zero
This is defined as 0 K, or –273.15 ^oC. This is the lowest temperature possible. In fact the third law of thermodynamics states that it is impossible to actually reach this temperature. When an object is cooled its internal energy is reduced and the temperature approaches absolute zero it becomes more and more difficult to lower the temperature further. We are always left with what is known as 'zero point energy' and so we can define absolute zero as the temperature at which substances have a minimum internal energy.

Celsius scale
The Celsius scale is now defined by q = T - 273.15.
The two fixed points on this scale are the ice point (0 ^oC) and the steam point (100 ^oC). The ice point and the triple point differ by 0.01 K.