Source: Jennifer Roushar; Images from http://www.kentchemistry.com/links/Matter/Phasediagram.htm
On a phase diagram, you will see the labels triple point and critical point. The triple point is the point where all three states of matter (solid, liquid, and gas) can be found together. Only at this temperature and pressure will you find all 3 states.
The critical point represents the temperature at which if exceeded the substance will always be a gas no matter what the pressure is.
Source: Jennifer Roushar; http://www.kentchemistry.com/links/Matter/Phasediagram.htm