The term that describes the differences in concentrations of a substance across cell membranes is concentration gradient.

Here's a breakdown:

* Concentration: The amount of a substance present in a given volume.

* Gradient: A gradual change in a value over a distance.

* Concentration gradient: The difference in concentration of a substance between two areas, such as across a cell membrane.

This difference in concentration drives the movement of substances across the membrane, a process known as diffusion.