The rate of change in the total hydraulic head per unit distance of flow in a given direction .
The slope of the water table or piezometric surface. This is the change in the hydraulic head over a certain distance. Mathematically it is the difference in hydraulic head over a distance along the flow path between two points.
Hydraulic gradient provides an indication of the direction of groundwater flow.
Source: www.ust.smoothstone.com/groundwater_intro