The term "core" refers to the muscles in the abdominals, hips, lower back and pelvis. When an exercise is performed that utilizes two or more of these muscle groups at the same time, that is considered a "core" exercise. Performing core exercises are especially important to athletes that require balance and coordinated muscular function such as swinging a baseball bat or golf club.
Examples of core exercises:
A unique feature of many core exercises is that there is no need for any special equipment to perform them as they often use one's own bodyweight as means to create resistance. This is particularly advantageous because the muscle that builds up and strengthens will be lean and defined based on one's own body weight.