The Collaborative International Dictionary of English v.0.48:

Priapism \Pri"a*pism\, n. [L. priapismus, Gr. ?, from Priapus
   the god of procreation, the penis, Gr. ?: cf. F. priapisme.]
   (Med.)
   More or less permanent erection and rigidity of the penis,
   with or without sexual desire.
   [1913 Webster]

WordNet (r) 3.0 (2006):

priapism
    n 1: condition in which the penis is continually erect; usually
         painful and seldom with sexual arousal