Engine Pressure Ratio. It is the Pressure behind the big fan divided by the pressure in front. It gives numbers something like 1.05 up to 1.78 on a B747 RB211 engine. There is no practical difference between measuring by this way or using N1, N2, N3 or whatever. Rolls Royce seems to prefer EPR and GE and Pratt N1. It makes no difference to the pilot.