Answer:
Gradient.
Explanation:
Psychology may be defined as the field of biology that mainly deals with the study of the mind, behavior and social aspect of the individual. The one who is expertise in the psychology is known as psychologist.
The psychological gradient shows the unequal or difference in the physical pressure or chemical difference between the two points. This might also the difference in the mental activity and its response in the different actions like reading, writing and at its conclusion.
Thus, the answer is gradient.
