Absolute (thermodynamic) temperature scales
An absolute (or thermodynamic) temperature scale is a scale based on the phenomena underlying temperature, the movement of particles.
More specifically, absolute temperature scales have a common reference point (known as absolute zero). Absolute zero is the temperature at which the particles of a substance possess the lowest possible internal energy (i.e. there is no particle motion).
There can be no temperature that is lower than absolute zero. This point is derived through the laws of thermodynamics.
All other temperatures are extrapolated linearly upwards from absolute zero.
Absolute scales may differ in unit values (the size of steps between units).
The Kelvin and the Rankine scales are the only absolute scales in use today.
The Celsius and Fahrenheit temperature scales are empirical scales that were linearly extrapolated to cover the whole range of possible temperatures described by an absolute scale.