The concentration of molecules or ions in a particular area as compared to another area is referred to as the concentration

Concentration gradient

Explanation:

When the concentration of molecules and ions is compared to the concentration of ions and molecules in another area, the difference in the concentration will be called the concentration gradient.

The concentration gradient is the factor which determines the rate and direction of the molecules and ions in the environment which could be physical or biological.

Thus, concentration gradient is the correct answer.