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An italic k, often subscripted B, relating temperature to energy.
Also known as: k, Boltzmann's constant
k_B converts a temperature into an energy per particle. Its subscript exists because k alone is badly overloaded — spring constants, rate constants and wavenumbers all claim the same letter.
Relates the average kinetic energy of particles to the thermodynamic temperature of the system they are in.
Contexts: Mathematics, Laboratory
Has had an exact value since 2019, which is how the kelvin is now defined — by fixing this constant rather than by the triple point of water.
Contexts: Mathematics, Laboratory
Carries a subscript B in careful writing because a bare k is claimed by spring constants, reaction rate constants and wavenumbers in the same literature.
Contexts: Mathematics, Laboratory
A lower-case Latin letter k in italic type, usually followed by a small raised-and-lowered capital B set as a subscript.
Both are a bare lower-case k, and both appear in physical formulae.
The kelvin is defined by fixing this constant.