The Collaborative International Dictionary of English v.0.48:

Diarrhea \Di`ar*rhe"a\, Diarrhoea \Di`ar*rh[oe]"a\,
   (d[imac]`ar*r[=e]"[.a]), n. [L. diarrhoea, Gr. dia`rroia, fr.
   dia`rrei^n to flow through; dia` + "rei^n to flow; akin to E.
   stream. See Stream.] (Med.)
   A morbidly frequent and profuse discharge of loose or fluid
   evacuations from the intestines, without tenesmus; a purging
   or looseness of the bowels; a flux. Diarrheal

WordNet (r) 3.0 (2006):

diarrhoea
    n 1: frequent and watery bowel movements; can be a symptom of
         infection or food poisoning or colitis or a
         gastrointestinal tumor [syn: diarrhea, diarrhoea,
         looseness of the bowels, looseness]