The Collaborative International Dictionary of English v.0.48:

Tenesmus \Te*nes"mus\, n. [NL., fr. Gr. ?, fr. ? to stretch: cf.
   L. tenesmos.] (Med.)
   An urgent and distressing sensation, as if a discharge from
   the intestines must take place, although none can be
   effected; -- always referred to the lower extremity of the
   rectum.
   [1913 Webster]

   Vesical tenesmus, a similar sensation as to the evacuation
      of urine, referred to the region of the bladder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tenesmus
    n 1: painful spasm of the anal sphincter along with an urgent
         desire to defecate without the significant production of
         feces; associated with irritable bowel syndrome