According to NSCA’s Essentials of Personal Training, an exercise is classified as a Core Exercise if a) it involves two or more primary joints, which would make it a multi-joint exercise and b) engages large muscles while activating synergistic muscles.
One multi-joint large muscle exercise (bench press) has the potential to activate as many as 4-8 small muscle, single joint exercises, single joint assistance exercises (front dumb bell raise).
If the core exercise places load on the spine (squat), it is further classified as a structural exercise. These exercises require that the torso be erect, ideally in a neutral spine position.
If structural exercises are performed quickly (power clean), they are classified as power or explosive exercises.
These types of exercises are very effective because they involve a lot of muscle mass and result in a large caloric expenditure.