Boltzmann constant

Fundamental physical constant: $k = R L = 1.380 6504 24 × 10 −23 J K −1$ , where $R$ is the gas constant and $L$ the Avogadro constant.

Source:

PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 963