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

This gradient is essential for many biological processes, such as:

* Passive transport: Substances move from an area of high concentration to an area of low concentration down the concentration gradient, without requiring energy.

* Active transport: Substances move against the concentration gradient, requiring energy to move from a low concentration area to a high concentration area.

Let me know if you'd like to know more about concentration gradients or any other biological terms!