"External rotation" refers to a movement or position where a body part, such as a limb or joint, turns or rotates outward away from the center or midline of the body. Full definition
People with shoulder issues sometimes can't do back squats, especially if you have trouble with external rotation. (getthisstrength.com)
You can perform external rotation exercises from many positions, and one of the classic positions is lying on your side. (all-about-arm-exercises.com)
In this video I go over a question I received on how much shoulder external rotation one should have during a kettlebell military press. (riseabovestrength.com)