Absolute Zero also known as 0 degrees Kelvin or -273.15 degrees Celsius describes the theoretical temperature at which particles have zero thermal energy- meaning that velocity of the particles goes to zero. Thus the velocity of the particles corresponds to temperature.
Absolute Zero was, previously thought to be the temperature at which an ideal gas would decrease to zero volume. At lower temperatures the velocity of the particles is lower which corresponds to lower volume. It now finds use as the bottom of an absolute temperature scale.
The Low Temperature Laboratory in the Helsinki University of Technology, Finland reached 2 x 10^-9 degrees Kelvin in 1989.