Final answer:
The variation in concentration of a substance from one region to another is known as a concentration gradient, which is closely associated with the process of diffusion, a type of passive transport where substances move down their concentration gradient.
Step-by-step explanation:
When the concentration of a substance varies from one region to another, it's called a concentration gradient. This concept is a fundamental aspect of the process of diffusion, where molecules move from an area of high concentration to an area of low concentration. Diffusion is a type of passive transport because it does not require energy expenditure by the cell. This process will continue until the concentrations on both sides of a membrane are equal, resulting in a state known as dynamic equilibrium. Examples of diffusion include the spreading of a perfume scent in a room or the dispersion of sugar in tea.