Overload refers to a training stress or intensity that is greater than what an individual is used to. If a program fails to adhere to the principal of overload, it will produce limited results. The most common methods of inducing overload include increasing the weight lifted, increasing the number of repetitions or sets of a given exercise, shortening the rest interval between sets, and increasing the number of training sessions per week.
While overload is an essential component of a resistance training program, one should apply overload systematically and should adhere to the principals of progression and variation. Overload must be progressive to allow sufficient time for the individual to adapt to the new training stimulus.
This information comes from NSCA’s Essentials of Personal Training by Coburn & Malek.