The phrase "medial head" refers to a specific part of something (such as a muscle or bone) that is located in the middle or towards the center of the body or structure. Full definition
Since it's done with a straight leg, it primarily targets the gastrocnemius, putting tension on both the lateral and medial heads of the muscle and giving them thickness and definition. (fitnessandpower.com)
The lateral and medial heads originate from the lateral and medial sides (respectively) of the distal posterior femur. (kingofthegym.com)
The lateral and medial head start under the shoulder and end at the elbow, while the long head runs from the shoulder blade and attaches to the elbow. (fitnessandpower.com)