It has units of pressure, and may be defined as pressure times its namesake coefficient, usually symbolized phi.  
The Lewis/Randall rule states that this quantity's value for each species in an ideal solution is proportional to the mole fraction of that species.  Use of this quantity corrects for nonideality due to attractive and repulsive forces between particles, and sidesteps problems with using the chemical potential for real fluids.   It has units of pressure, and may be defined as pressure times its namesake coefficient, usually symbolized phi.  For an ideal gas, phi is equal to one, and this quantity equals the pressure.  FTP, what is this quantity which represents the tendency of a substance to escape from its phase?
The Lewis/Randall rule states that this quantity's value for each species in an ideal solution is proportional to the mole fraction of that species.  
fugacity