Supination is a rotating movement of the forearm. The forearm has two bones: the radius and the ulna (formerly called the cubitus).
This movement consists in turning the forearm in such a way that, when it is parallel to the ground, the hand's palm faces upwards. This is therefore an external rotation of the forearm. During the supination movement the two bones of the forearm, which are crossed, become parallel.
The opposite movement to supination is pronation. All of these antagonistic movements are referred to as pronosupination.
Supination is a rotating movement of the forearm. © Taleb247, Flickr, public domain