Boltzmann constant, \(k\)

https://doi.org/10.1351/goldbook.B00695
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).
Sources:
BIPM, The International System of Units, SI Brochure, 9th ed. (2019) (http://www.bipm.org/en/publications/si-brochure/)
Newell et al., Metrologia 55, L13-L16 (2018) (https://doi.org/10.1088/1681-7575/aa950a)