Critical fluid

When a liquid is heated in closed vessel, boiling does not occur.
As the diagram shows, when the temperature is increased the density of the liquid phase decreases, and the density of the vapour phase increases.

At and above the critical temperature a single phase fills the container. 
This is known as the supercritical fluid.

As long as the temperature is above the critical temperature, increasing the pressure on the supercritical fluid does not cause condensation of the liquid phase.

Supercritical fluids are like gases in that they totallly fill the volume of the container.  They are like liquids in that they can act as a solvent.