Any adjustment you make to a specific exercise that allows you to more safely complete the move, account for physical limitations/pain, or slowly build up to the full version...all while maintaining the goal of the exercise. For example, doing pushups from your knees.
It is important to realize WHY you are modifying an exercise and how you are doing so. Some modifications will take pressure off of one area, but put more work on another. It is best to seek out the help of a trainer to at the very least help you design an appropriate and safe exercise program. Common reasons to modify exercises include a client's inability to maintain good form, limitations in range of motion of a joint, or pain/discomfort when attempting the exercise.
Not all modifications are created equal. It can be easy to see someone struggling with an exercise and say, "they need a modification". But the next part is where a qualified professional is necessary. A deep understanding of normal human movement is necessary to recognize and rehabilitate abnormal movement. When modifying, it is important to determine the area of difficulty (are push-ups too difficult because the person lacks core strength, upper body strength, or maybe painful wrists from repetitive motion problems?) and what alternative positions can remove some strain on the relevant area while allowing the same benefits/strengthening as the full version of the exercise. After all, the purpose of modification is to allow a build up in strength and/or endurance until the complete exercise can be performed.
I will be offering Fascial Stretch Therapy in the Anchorage, Alaska area starting in early September. This service is provided with the client standing, on the floor, and/or massage table fully clothed. It can be very useful to correct imbalances, reduce pain, and maintain physical fitness. For more information feel free to check out the Stretch to Win Institute Blog.