noun pl. mediastina -·na (-nə)

1. a membranous partition between two cavities of the body, esp. that separating the lungs or the two pleural sacs
2. the space between the pleural sacs, containing the heart and other chest viscera except the lungs

Etymology: ModL < ML mediastinus, in the middle (form infl. by L mediastinus, servant, medical assistant) < L medius, middle: see mid