Rotation of a limb around its own long axis, inward toward the midline or outward away from it, most often coached at the shoulder and the hip.
At the shoulder, internal rotation turns the palm toward the body and external rotation turns it away. The internal rotators (lats, pecs, subscapularis) are trained by nearly every press and pull; the external rotators (infraspinatus, teres minor) are trained by almost nothing unless you program them.
That imbalance is why face pulls and band external rotations sit in so many upper-body sessions. Two sets of 15 at a weight the client can control through the full range, three times a week, is a maintenance dose. Loading it heavily defeats the point, since the muscles are small and the job is control.
At the hip, external rotation is what opens the knees in a squat. A client who cannot hold their knees out under load usually needs the cue and some direct glute medius work, not a wider stance.