Boltzmann constant; k (L. Boltzmann)
This is a constant that describes the relationship among temperature & kinetic energy for molecules in an ideal gas. This is equal to 1.380 622 x 10^{-23} J/K.
Boyle's law (R. Boyle; 1662); Mariotte's law (E. Mariotte; 1676)
At constant temperature the product of the pressure & the volume of an ideal gas is a constant.