Absolute zero
Absolute zero is the practical minimum of 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 thermal motion (i.e. the movement of molecules in a gas, atoms in a crystal) ceases.
Examples of Absolute Zero
No place in our universe can reach 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 Black-Body Radiation, which occupies the vacuum of space. This radiation keeps the universe at this temperature. The Cosmic Backround Explorer measured this value. It has since been theorized that this risidual heat energy is leftover from the Big Bang, the theorized beginning of our universe, although this has not been proven.
Reaching Absolute Zero
Scientists have not yet reached Absolute Zero, but have come quite close. The first example of this occured in 1908 when Kammerlingh Onnes liquified Helium. The temperature he reached was approximately four Kelivn. 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 picoKelvin.