A critical point is the point at which the boundary that separates two stable states of matter disappears. H2O, for instance has three states of matter: liquid, in the form of water, solid, in the form of ice and gas in the form of steam. This is a diagram of those phases, which in and of themselves are examples of emergence. As you can see, there are boundaries between these three phases depending on the pressure and temperature. When we look at the boundary between the liquid phase and the gas phase, it does not continue forever but stops at a critical point in the phase diagram. This critical point reflects the fact that at high pressure and high temperature, it is impossible to distinguish between the liquid and gaseous phases and H2O enters a different supercritical phase of matter.