For all practical purposes, absolute zero is the coldest temperature, but the complete story of absolute zero is a little more complicated.
Absolute zero is a temperature where molecular motion completing ceases in matter. There may still be motion at the quantum level but all molecules are frozen.
Measuring Temperature
We are all familiar with temperature scales, either Celsius or Fahrenheit. The Celsius scale marks it's zero point where water freezes at sea level. The boiling point of water at sea level is marked at 100 degrees. Fahrenheit marks the freezing point at 32 degrees. The size of a Fahrenheit degree is a little bit bigger than the size of a Celsius degree, making the Celsius scale a finer measurement tool. The Celsius and Fahrenheit scales are commonly used to measure temperatures at the macroscopic scale, on Earth.
Temperature Scales
Physicists and many other scientists use the Kelvin scale. Kelvin degrees are the same size as Celsius degrees making temperatures easy to convert between the two scales. There is another scale called the Rankine scale that uses Fahrenheit sized degrees. For both Rankine and Kelvin, the scale begins where there is no thermal motion, the coldest possible temperature. Zero degrees Kelvin is equal to -273.15 degrees Celsius and -459.67 degrees Fahrenheit.
The Coldest Place
It is generally believed to be impossible to obtain a temperature of absolute zero. Environments of absolute zero don't occur naturally in the present-day universe. The coldest natural location yet discovered is in the Boomerang nebula, 5000 light years away. It has a temperature of just 1 degree K, and is the only known area that is actually colder than the residual background radiation from the Big Bang.
Absolute Zero in the Lab
Scientists have been able to create environments only a few pK away from absolute zero. These environments are used to study matter at extremely cold temperatures. Matter can display some strange behavior such as superconductivity (no electrical resistance) and superfluidity (no viscosity) when heat is removed so completely and can tell us a lot about the nature of matter itself.
