Compressibility factor ($Z=p/(n{k}_{B}T)$)

The compressibility factor is the ratio $p/(n{k}_{B}T)$, where $n$ is number density. A classical ideal gas has $Z=1$. The virial expansion writes $Z=1+B_{2}n+B_3{n}^2+\cdots$ for a dilute gas; this dimensionless ratio is distinct from isothermal compressibility, which is a response derivative with units of inverse pressure.