Why is -273 C is called absolute zero?

Absolute zero is the theoretical temperature at which all substances have zero thermal energy. Originally conceived as the temperature at which an ideal gas at constant pressure would contract to zero volume, absolute zero is of great significance in thermodynamics and is used as the fixed point for absolute temperature scales. Absolute zero is equivalent to 0 degrees Kelvin, -459.67 degrees F, or -273.15 C.
The velocity of a substance's molecules determines its temperature; the faster the molecules move, the more volume they require, and the higher the temperature becomes. The lowest actual temperature ever reached was two-billionth of a degree above absolute zero. It was achieved by a team at the Low Temperature Laboratory in the Helsinki University of Technology, Finland, in October, 1989.
Having a limiting temperature has several thermodynamic consequences for example at 0 kelvin all molecular motion does not cease there is just no energy available for transference to other systems it is therefore more accurate to say that at absolute zero molecular energy is minimal if we say that all substances have zero energy this is a violation of the Heisenberg Uncertainty Principal so it is impossible to have zero energy.

The opposite of absolute zero is called Planck Temperature.

As of now it is the highest possible temperature that matter can exist at.
It is approximately 1.41679 x 10^32 Kelvin.
Beyond this temperature everything turns to energy, all subatomic particles break down and become individual black holes destroying space and time.
The universe has already reached this temperature once during the first instant of the Big Bang.