The Boltzmann constant k (also kB) is the ratio of the molar gas constant R to Avogadro's constant NA. It can be thought of as the gas constant for a single molecule (or even for an arbitrary particle in a colloidal solution) rather than for a mole.
For more info on this see the equipartition theorem.
According to NIST the Boltzmann constant has a value of 1.3806504 x 10-23 J/K with a standard uncertainty of 0.0000024 x 10-23 J/K and a relative uncertainty of 1.7 x 10-6 (this is represented by the concise form 1.380 6504(24) x 10-23 J/K
The Boltzmann constant can also be represented in alternative units as 8.617385 x 10-5 eV/K
- Fundamentals of Physics, Fourth Edition by David Halliday, Robert Resnick, and Jearl Walker p582