'Progressive overload' in exercise refers to the gradual increase of stress placed on the body during workouts to continue making fitness gains. This technique involves modifying various workout variables such as weight, repetitions, sets, and frequency to challenge the body beyond its current capabilities. By systematically increasing the demands of a training program, individuals can improve their strength, endurance, and muscle size, as the body adapts to respond to heightened stress levels. This principle is essential for long-term progress in any fitness regimen.