A cast was a medical device used to immobilize a body part which had sustained an injury, such as a broken bone. It encased the affected part with a solid material to prevent motion which might worsen the injury.
Gul Dukat placed a cast on Benjamin Sisko's broken arm upon their crash landing on a planet, following the destruction of the USS Honshu. He did so despite the presence of a bone regenerator on their shuttlecraft, as he claimed to lack the training to operate it. (DS9: "Waltz")