This is the unequal distribution of ions across a cell membrane.

Concentration gradient

Explanation:

Concentration gradient of the ions across the membrane generates the membrane resting potential.

Concentration gradient means that there is unequal distribution of the ions on different sides of membrane. For example, the concentration of K ions is much higher within the cell then out of the cell. Opposite is with the Na ions. When ions move from the area of their higher concentration to the are with the lower concentration, we say they move down the gradient or diffuse (no energy required). On the other hand, movement of ions against their gradient means that this process requires energy and involves protein pumps.