The Collaborative International Dictionary of English v.0.48:

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.
          [1913 Webster]

   2. (Zool.) Same as Pleuron.
      [1913 Webster]