Final answer:
The hydraulic gradient is the difference in hydraulic head divided by the horizontal distance between two points in groundwater geology.
Step-by-step explanation:
The difference in hydraulic head divided by the horizontal distance between two points in groundwater geology is called the hydraulic gradient. The hydraulic head is the potential energy per unit weight of water, and it represents the pressure that drives groundwater flow. The hydraulic gradient is a measure of the slope or steepness of the water table or potentiometric surface.