In fitness, "progressive overload" refers to the systematic increase of stress placed on the body during training. This principle is essential for muscle growth, strength gain, and overall fitness improvement. By gradually increasing weights, repetitions, or intensity, the body adapts to the higher demands, leading to physiological changes, such as increased muscle tissue and improved endurance. This concept ensures that workouts remain challenging and that athletes can continue to progress without plateaus, ultimately helping meet fitness goals effectively.