The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale referenced to absolute zero, the theoretical absence of all thermal energy. So by definition, the temperature of a substance at absolute zero is zero kelvin (0 K). The secondary reference point on the Kelvin scale is the triple point of water (0.01 degrees Celsius). The Kelvin scale is the difference between these two reference points, with the kelvin defined as one 273.16th of this scale. The Kelvin scale and the kelvin are named after the Belfast-born physicist and engineer William Thomson, 1st Baron Kelvin (1824ʹ1907), who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to as a "degree", nor is it typeset with a degree symbol; that is, it is written K and not °K. The kelvin and the degree Celsius are often used together, as they have the same interval, and 0 kelvin is V273.15 degrees Celsius.

reference: http://en.wikipedia.org/wiki/Kelvin

In general, 'degrees' are found in units which are mainly arbitrary. Like, degrees Fahrenheit and Celsius, which are both arbitrary in the sense that their origin is chosen in an arbitrary way, and they are meant to be convenient instead of absolute. The same goes for angle degrees, since that also divides the circle into 360 degrees arbitrarily.

When the units are absolute, or measured directly with respect to something else, one drops 'degrees' from the units. Like, there is no 'degrees' in radians, which is the 'natural' way of measuring angles. (One can appreciate the 'natural'ness of radians after one sees how sines and cosines work, and some series expansion of those, and their relations with the exponential function, and complex numbers...).

It is the same way for Kelvin. Temperature is defined in terms of the average energy of particles in a system, and Kelvin is directly proportional to that -- the zero in the Kelvin scale corresponds to absolute zero, and not any arbitrary temperature, and Kelvin is the 'natural' unit to measure temperature.

However, let me also note that degrees are still used with Kelvin in some sources, even textbooks, although it is generally agreed that they should not.

by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey reference: http://www.physlink.com/education/askexperts/ae129.cfm

**arbitrarily - randomly: in a random manner; "the houses were randomly scattered"; "bullets were fired into the crowd at random" reference: wordnetweb.princeton.edu/perl/webwn

arbitrary (adj) - based on or determined by individual preference or convenience rather than by necessity or the intrinsic nature of something reference: Merriam-Webster͛s Online Dictionary

The Kelvin temperature scale is sometimes called the absolute temperature scale, especially in older books. It was developed by William Thomson, also known as Lord Kelvin, in 1848. A Kelvin degree is the same size as a Centigrade degree. This temperature scale however uses absolute zero, rather than the freezing point of water, as the zero point. In this temperature scale water freezes at 273.15 Kelvins and boils at 373.15 Kelvins. The Kelvin temperature scale should be used in thermodynamic calculations. Its principle difference is that kelvin measurements, written as K have a much lower starting point: 0K or 0 Kelvin (note the absence of the degree symbol °).

It bears additional mention that kelvins are not measured by degrees. They were considered so until 1968, when the 13th General Conference on Weights and Measures determined to drop the degree reference. This decision was made because Thomson͛s measurement referred to an absolute and specific temperature (where no heat energy exists). Celsius, conversely, uses the point of reference of water freezing at the bottom of its scale, and this doesn͛t accurately account for heat energy left in the water at this point (273.15 K). Instead, these temperature units are thought of as kelvins. When you measure something by Celsius, for example the boiling point of water, you are measuring in degrees (approximately 100° C). The boiling point of water on Thomson͛s scale is approximately 373 kelvins or written as 373 K.

There are some important marking points for Thomson͛s scale. Absolute zero is 0 K, and the triple point of water, where water can exist as gas, liquid and solid is 273.16 K (.01° C or 32.018° F). The melting point of ice, 0° C or 32°F, is 273.15 K. The boiling point of water, approximately 100° C or 212° F, is exactly 373.1339 K. The scientific community often uses kelvin and Celsius measurements interchangeably or at the same time. You may see data on temperature given both a C degrees measurement and a kelvin measurement. This is especially the case when discussing heat energy units between the melting point of ice and absolute zero.

reference: http://answers.yahoo.com/question/index?qid=20090810044912AAnPr32

1967/1968

Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree absolute", symbol °K. Furthermore, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."

reference: http://en.wikipedia.org/wiki/Kelvin