A constant which describes the relationship between temperature and kinetic energy for molecules in an ideal gas. It is equal to 1.380622 x 10-23 J/K.
The Boltzmann constant (k or kB) is the physical constant relating energy at the individual particle level with temperature, which must necessarily be observed at the collective or bulk level. It is the gas constant R divided by the Avogadro constant NA.
Boltzmann's grave in the Zentralfriedhof, Vienna, with bust and entropy formula.
Source: wikipedia, NASA