The Collaborative International Dictionary of English v.0.48:

Aposteme \Ap"os*teme\, n. [L. apostema, Gr. ? the separation of
   corrupt matter into an ulcer, fr. ? to stand off: cf. F.
   apost[`e]me. See Apostasy.] (Med.)
   An abscess; a swelling filled with purulent matter. [Written
   corruptly imposthume.]
   [1913 Webster]