The terms drive theory and drive reduction theory refer to a diverse set of motivational theories in psychology. Drive theory is based on the principle that organisms are born with certain physiological needs and that a negative state of tension is created when these needs are not satisfied. When a need is satisfied, drive is reduced and the organism returns to a state of homeostasis and relaxation. According to the theory, drive tends to increase over time and operates on a feedback control system, much like a thermostat.
See also 
- Drive theory (Wikipedia)