The Collaborative International Dictionary of English v.0.48:

Thoracentesis \Tho`ra*cen*te"sis\, n. [NL., fr. Gr. ? thorax + ?
   pricking, from ? to prick, stab.] (Surg.)
   The operation of puncturing the chest wall so as to let out
   liquids contained in the cavity of the chest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thoracentesis
    n 1: removal of fluid from the chest by centesis for diagnostic
         or therapeutic purposes [syn: thoracocentesis,
         thoracentesis]