R is used to designate the
universal gas constant. E.g. in the
ideal gas law: PV = nRT. P is
pressure, V is
volume, n is number of
moles of gas, T is
temperature, and R is 8.3145 J/mol-K = 0.0831454 L-bar/mol-K = 0.082058 L-atm/mol-k. The value of R is experimentally determined in the "
ideal-gas range" (low pressure, high volume).
As a side note, R is always given per mole. If divided by
Avogradro's Number to make it per molecule, it becomes the
Boltzmann constant = 1.38066e-23 J/K.