(K) The SI unit of temperature.
Definition: One Kelvin is 1/273.16 of the difference between the triple point of water (at exactly 273.16 K) and absolute zero.
The triple point of water is the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states. Absolute zero is the temperature at which all molecular motion discontinues. 0 K is according to -273.15° Celsius and -459° Fahrenheit.

A metric unit of temperature.
Definition: One degree is 1/273.16 of the difference between the triple point of water (at exactly 0.01 °C) and absolute zero.
The triple point of water is the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states. Absolute zero is the temperature at which all molecular motion discontinues.
The Celsius temperature scale is named for the Swedish astronomer and physicist Anders Celsius (1701-1744), who used a similar scale.

See also Kelvin.

The international system for units.
Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.
It is based upon 7 principal units:
Length - meter (m)
Mass - kilogram(kg)
Time - second (s)
Electric current - ampere (A)
Temperature - Kelvin (K)
Amount of substance - mole (mol)
Luminous intensity - candela (cd)
From these basic units many other units are derived and named.

See also Ultrasound Physics.