A critical point, also known as a critical state, occurs under substance-specific sets of conditions, (such as specific conditions of temperature, pressure or composition),  and under these substance-specific conditions no phase boundaries exist. The state of the substance is specified by some number of state variables, like gases are described by pressure, temperature and volume.  This table lists critical temperatures, critical pressures, and corresponding critical molar volumes, for various substances and these are often used in predictive state equations for real gases.

State Variable ¹

A thermodynamic system is described by a number of thermodynamic parameters (e.g. temperature, volume, pressure) which are not necessarily independent. The number of parameters needed to describe the system is the dimension of the state space of the system (D).  For example, a monatomic gas having a fixed number of particles is a simple case of a two-dimensional system ( D = 2 ). In this example, any system is uniquely specified by two parameters, such as pressure and volume, or perhaps pressure and temperature.

1. ^ http://en.wikipedia.org/wiki/State_function