The Kelvin:

The SI unit of temperature is the Kelvin, represented by K (uppercase and non-italicized). It is a quantify of how much heat exists relative to absolute zero, that symbolizes the absence of all heat and that is therefore the coldest possible temperature. A temperature of 0 K symbolizes absolute zero. Officially, the Kelvin is defined as a temperature rise (an increase or decrease) of 0.003661 portion of the thermodynamic temperature of the triple point of pure water. The pure water at sea level freezes or melts at 273.15 K & boils or condensed at 373.15 K.

What, you may ask, is the meaning of triple point? In the situation of water, it is almost exactly similar as the freezing point. For water, it is the pressure and temperature at which it can exist as liquid, vapor, and ice in balance. For practical reasons you can think of it as freezing.