Pleura \Pleu"ra\, n.; pl. L. {Pleur[ae]}, E. {Pleuras}. [NL., n. fem., fr. Gr. ? a rib, the side.] 1. (Anat.) (a) The smooth serous membrane which closely covers the lungs and the adjacent surfaces of the thorax; the pleural membrane. (b) The closed sac formed by the pleural membrane about each lung, or the fold of membrane connecting each lung with the body wall. 2. (Zo["o]l.) Same as {Pleuron}.