Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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