gas constant
When you write the ideal gas law as PV = nRT, the letter R is the number that makes the two sides balance — the conversion factor that translates between the everyday units of pressure, volume and temperature on one hand and amount of gas on the other. It is called the gas constant.
Its defining feature is that it is universal: the same value of R works for every gas — hydrogen, oxygen, carbon dioxide, all of them. In one common set of units R is about 8.314 joules per mole per kelvin. Because it ties pressure-times-volume (an energy) to amount times temperature, R is really an energy-per-mole-per-degree, a bridge between mechanical and thermal quantities.
The gas constant matters far beyond gases. It reappears throughout thermodynamics and physical chemistry — in the equations for reaction equilibrium, electrode potentials and reaction rates. It is also closely related to the Boltzmann constant: R is simply Boltzmann's constant scaled up from a single molecule to a whole mole.
The same R that fixes the size of a balloon through PV = nRT also appears in the Arrhenius equation for reaction rates and the Nernst equation for batteries — one constant quietly threading through much of physical chemistry.
One constant, R, recurs across gases, kinetics and electrochemistry.
R takes different numerical values depending on the units you choose for pressure and volume (for example 0.0821 in litre-atmosphere units, 8.314 in joules). Always match the value of R to the units in your problem.