1. The effective stress on a rock, the difference between the pressure of a rock’s pore fluid and that produced by the weight of the overburden. It is important in drilling boreholes, controlling compaction, and determining seismic velocity. Also called Terzaghi effective stress. See normal pressure. 2. What drillers mean by differential pressure is usually the difference in fluid pressures across the borehole wall, the difference between the pressure induced by the mud column and that of the formation fluid.