1. n. 
The change in pore pressure per unit of depth, typically in units of psi/ft or kPa/m. Pressure increases predictably with depth in areas of normal pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 psi/ft, or 9.792 kPa/m, and 0.465 psi/ft for water with 100,000 ppm total dissolved solids (a typical Gulf Coast water), or 10.516 kPa/m. Deviations from normal pressure are described as high or low pressure.
2. n. 
A change in pore pressure as a function of distance. This can refer to radial change in pore pressure with distance from the well (which can be calculated from well-test analysis results) or to change in pore pressure with depth (which can be measured by formation tests, and implies formation fluid density and/or fluid contacts).