Somehow I am worried it is a can of worms, but I will still try. This started with a student confused with statements describing critical temperature for the gas/liquid equilibrium. Some books and sources state it is the highest temperature AT which it is possible to liquefy the gas just by applying higher pressure, some state gas can be liquefied only BELOW the critical temperature, I have also seen a statement that the critical point is the first point where only one phase (supercritical fluid) exists. While it doesn't matter much for practical purposes, I can see how it is confusing. So, which one it is? At, or below? My thought was - critical point is the end point of a phase equilibrium curve. In other words - as long as the curve exists there are two phases, liquid and gas. But, if we think about the supercritical liquid as a third phase, critical point becomes something like a triple point, where three phases - gas/liquid/supercritical liquid - coexist (yes, I know - it is a moot, as at critical point these three phases become indistinguishable). This makes whole discussion much easier and less confusing IMHO, and it also makes the answer "at" the correct one. But is it?