For the forearm, when standing in the anatomical position pronation will move the palm of the hand from an anterior-facing position to a posterior-facing position without an associated movement at the shoulder (gleno-humeral joint). For the foot pronation will cause the sole of the foot to face more laterally than when standing in the anatomical position.
Pronation is the opposite of supination.
Pronation of the forearm should not be confused with medial rotation of the shoulder. When standing in the anatomical position, medial rotation of the shoulder can perform part of the movement to change the palm from facing anteriorly to posteriorly. Pronation of the forearm occurs at the radioulnar joint so such a movement does not constitute it. To isolate medial rotation in the shoulder from pronation of the forearm one can stand with one's elbows tucked against the trunk, with the elbows flexed to 90 degrees with the forearms pointing directly in front of the body with the palms of the hands facing upwards. Keeping ones elbows tucked at the sides, the act of moving one's forearms so that the palms are facing downwards is pronation of the forearm.
The pronated foot is one in which the heel bone angles inward and the arch tends to collapse. A "knock-kneed" person has overly pronated feet. This flattens the arch as the foot strikes the ground in order to absorb shock when the heel hits the ground, and to assist in balance during mid-stance. If habits develop, this action can lead to foot pain as well as knee pain, shin splints, achilles tendinitis, posterior tibial tendonitis, and plantar fasciitis.