The errors in ideal gas predictions become most striking as you approach the gases critical conditions.
At higher temperatures and/or higher pressures the difference between a gas and a liquid eventually disappears(!) and a supercritical fluid is formed. The point at which this happens is called the critical point
The highest temperature at which a species can coexist as a liquid and a vapor is termed its critical temperature, Tc.
The highest pressure at which a species can coexist as a liquid and a vapor is termed its critical pressure, Pc.
A supercritical fluid is a substance which is above its critical temperature, Tc or pressure, Pc.
One way to envision the behavior of a material is to use a phase diagram. It is often the case that we plot P vs. T in order to show the phase behavior of the material:
A phase diagram is a plot of one system variable versus another which shows the conditions at which the substance is a solid, liquid, gas, or s.c. fluid.
If vapor and liquid are coexisting then setting either T OR P determines the conditions...we will look at this further in a minute.
On the vapor-liquid line the temperature is called the boiling point and the pressure is the vapor pressure.
On the vapor-solid line the temperature is called the sublimation point.
On the liquid-solid line the temperature is called the freezing/melting point.
Sketch a phase diagram and label relevant regions/points (solid, liquid, gas, critical point, etc.)