A kelvin (K) is a (SI) unit of temperature, generally used to indicate an amount of temperature above absolute zero, i.e., above the physical lower bound on temperatures. The unit size is borrowed from the Celsius temperature scale (which is pegged at zero for water's freezing temperature and 100 for water's boiling point) but among SI efforts is a more careful definition, based upon measurable physical values such as the speed of light and the Planck constant. Unless explicitly referring to a temperature difference, a number of kelvins refers to that number of kelvins above absolute zero. Physical relations tend to be based upon temperature scaled from absolute zero and kelvins are a convenient scale in the sciences.
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Regarding writing style, the unit kelvin is not capitalized, but the abbreviation K is capitalized: not the usual practice for SI units, but presumably a means to avoid potential confusion with "k for kilo". The term Kelvin scale for referring to the temperature scale in kelvins is capitalized. Terms like "MK" for millions of kelvins (or "kK" for thousands) are used, but it seems to me they are often avoided and readers aren't used to them: mK and μK seem less avoided when referring to very low temperatures. Unlike Fahrenheit and Celsius/centigrade (and Rankine), kelvins are not referred to as degrees, i.e., you do not write 100°K but 100 K. Referring to kelvins as degrees was formerly the practice, but SI standardization efforts changed this in 1967.