Absolute zero is the minimum temperature, defined as 0 Kelvin on the absolute temperature scales (the zero on the Kelvin scale has had to adjust, slightly, as measurements of absolute zero are refined). This corresponds to -273.15 degrees Celsius and -459.67 degrees Fahrenheit. At absolute zero, all motion (i.e. the movement of molecules in a gas, atoms in a crystal) would cease.
No place in our universe is at absolute zero, but there are places that get close. Space is approximately 2.73 kelvin. This is not because heat from stars in the universe keep the temperature up, but because of cosmic background radiation. The Cosmic Background Explorer (CBE) measured this value. The Big Bang theory predicts a leftover background radiation and that such radiation would have a relative uniformity, which is reinforced by the evidence presented by the CBE's observations. However, these observations cannot account for the horizon problem.
Reaching Towards Absolute Zero
It is considered theoretically impossible to achieve absolute zero, but scientists have come quite close. The first example of this occurred in 1908 when Kammerlingh Onnes liquefied Helium. The temperature he reached was approximately four kelvin. Lower temperatures have been reached with different isotopes of Helium. A temperature of 0.3 kelvin has been reached with 3He. The lowest temperature ever reached by any substance was 280 picokelvins.