The difference in the concentration of a substance from one location to another is called a

Concentration gradient

Explanation:

The larger the difference in concentration of the molecules between the two locations, the higher the concentration gradient. The system is therefore referred to as having a high Free Gibbs Energy (and low entropy), which is the potential energy intrinsic of the system to do work. Therefore, it will be spontaneous that the molecules will want to move from the location of higher concentration to the location of lower concentration until equilibrium is achieved (process called difussion).