Critical Point

The liquid/gas line of a phase diagram slopes up to the right. One way to think about this is that as molecules move faster, the attractions they have for each other become less effective (South Street Effect). But, if the gas is compressed enough, the molecules will bump into each other so often that they will eventually start to stick together and become a liquid.

However, if the temperature gets high enough, the particles are moving so fast, that they will act independently even if they are packed tightly together. This temperature is called the critical temperature. At that temperature, there is no visible distinction between the two phases.

A definition of critical temperature would say something like this:

The critical temperature is the temperature, above which, a gas cannot be liquefied by increasing pressure.

At that temperature, the liquid gas line becomes, essentially, a vertical line with all areas to the right of that line occupied by the gas phase.

The point on the graph where the liquid gas line reaches the critical temperature and becomes a vertical line is called the critical point. And the temperature which that happens is called the critical temperature.