The Collaborative International Dictionary of English v.0.48:

Empyema \Em`py*e"ma\, n. [NL., fr. Gr. ?, from ? to suppurate; ?
   in + ? pus.] (Med.)
   A collection of blood, pus, or other fluid, in some cavity of
   the body, especially that of the pleura. --Dunglison.
   [1913 Webster]

   Note: The term empyema is now restricted to a collection of
         pus in the cavity of the pleura.
         [1913 Webster]

WordNet (r) 3.0 (2006):

empyema
    n 1: a collection of pus in a body cavity (especially in the
         lung cavity)