The term that describes the difference in concentration of a substance across a cell membrane is concentration gradient.

Here's why:

* Concentration: Refers to the amount of a substance in a given volume.

* Gradient: Indicates a change in a value over a distance.

Therefore, a concentration gradient means that the concentration of a substance is higher on one side of the cell membrane compared to the other side. This difference in concentration drives the movement of molecules across the membrane, either passively (like diffusion) or actively (requiring energy).