The virial equation of state for one mole of a real gas is written as $Z=1+\frac{B}{V_m}+\frac{C}{V_m^2}+\dots$. The coefficient $B$ is called the

The leading term 1 is sometimes called the first virial coefficient; $B$, which multiplies $1/V_m$, is the second virial coefficient and $C$ the third. $B$ depends on temperature only and is the most important correction at moderate pressures.