molar heat capacity
How much heat it takes to warm one mole of a substance by one kelvin. It is like specific heat capacity, but reckoned per mole instead of per gram, which makes it especially natural for comparing gases molecule for molecule.
Precisely, C = Q / (n dT), in units of J/(mol·K). For a gas there are two versions: at constant volume, C_V, where all the heat becomes internal energy, and at constant pressure, C_P, where the gas also does expansion work as it pushes outward. They differ by exactly R: C_P - C_V = R (Mayer's relation). Their ratio, gamma = C_P / C_V, is called the adiabatic index.
Values: a monatomic ideal gas has C_V = (3/2)R, C_P = (5/2)R, and gamma = 5/3; a diatomic gas at room temperature has C_V = (5/2)R and gamma = 7/5. Because these depend on the degrees of freedom, measuring gamma actually reveals the molecular structure.
Helium (monatomic) has gamma of about 1.67, while air (mostly diatomic nitrogen and oxygen) has about 1.40.
The heat needed to warm one mole by 1 K.
C_P is always larger than C_V for a gas because at constant pressure some of the heat goes into expansion work rather than into raising the temperature.