PR is the ratio of the pressure to the critical pressure.

Ratio of the pressure of a gas to its critical pressure, in like units. ( 060)

The pressure leaving the outlet opening of a valve (P2 ). More commonly called outlet pressure.

The calculated value of atmospheric pressure at mean sea level or some other specified level, as derived (reduced) from station pressure or actual pressure. Thus, sea level pressure is nearly always a reduced pressure. See reduction.

In thermodynamics, the reduced pressure of a fluid is defined as its actual pressure divided by its critical pressure.