compressibility factor
Suppose you want to know, at a glance, how far a real gas has wandered from ideal behaviour. The compressibility factor is exactly that scorecard: a single number, written Z, that says whether the gas is behaving like an ideal gas, and if not, in which direction it is straying.
It is defined as the ratio of what the gas actually does to what an ideal gas would do under the same conditions — formally, Z equals PV divided by nRT. For a perfectly ideal gas Z is exactly one. When attractions between molecules dominate, the gas is more compressible than ideal and Z dips below one; when the molecules' own bulk dominates at high pressure, the gas resists compression and Z rises above one.
The compressibility factor matters as the practical language of real gases. Plotting Z against pressure for a given gas shows at a glance where the ideal gas law is safe and where it would mislead you — essential knowledge in engineering gas pipelines, storage tanks and any high-pressure process.
For nitrogen at moderate pressures Z dips just below one, signalling that attractions make it slightly easier to compress than ideal; squeeze it to hundreds of atmospheres and Z climbs above one as the molecules' own size takes over.
Z below one means attraction wins; Z above one means molecular size wins.
Z is not a fixed property of a gas; it changes with both temperature and pressure. The same gas can have Z below one in one set of conditions and above one in another, and every gas approaches Z equal to one as pressure drops toward zero.