The **kelvin** is the base unit of temperature in the International System of Units (SI), having the unit symbol K. It is named after the Belfast-born Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).

The kelvin is now defined by fixing the numerical value of the Boltzmann constant k to 1.380 649×10^{−23} J⋅K^{−1}. This unit is equal to kg⋅m^{2}⋅s^{−2}⋅K^{−1}, where the kilogram, metre and second are defined in terms of the Planck constant, the speed of light, and the duration of the caesium-133 ground-state hyperfine transition respectively.^{[1]} Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously, such as the International Prototype of the Kilogram, whose mass diverged over time from the original value.

One kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380 649×10^{−23} J.^{[2]}

The **Kelvin scale** fulfills Thomson's requirements as an absolute thermodynamic temperature scale. It uses absolute zero as its null point.

Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or written as a degree. The kelvin is the primary unit of temperature measurement in the physical sciences, but is often used in conjunction with the degree Celsius, which has the same magnitude.

In electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate noise floor, i.e. the noise temperature. The so-called Johnson–Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit using the Friis formulas for noise.

The symbol is encoded in Unicode at code point U+212A K U+212A K KELVIN SIGN. However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using U+004B K LATIN CAPITAL LETTER K instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: U+2126 Ω OHM SIGN, U+212A K KELVIN SIGN, and U+212B Å ANGSTROM SIGN. In all three instances, the regular letter should be used."^{[19]}