Boltzmann constant, \(k\)
The Boltzmann constant, \(k\), is one of seven fixed constants defining the International System of Units, the SI, with \(k = 1.380\ 649\times 10^{-23}\ \text{J K}^{-1}\). The Boltzmann constant is a proportionality constant between the quantities temperature (with unit kelvin) and energy (with unit joule).
BIPM, The International System of Units, SI Brochure, 9th ed. (2019) [Terms] [Book]
Newell et al., Metrologia 55, L13-L16 (2018) [Terms] [Book]