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):

diarrhea
    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]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "diarrhea":
   BM, abscess, ague, anemia, ankylosis, anoxia, apnea, asphyxiation,
   asthma, ataxia, atrophy, backache, bleeding, blennorhea,
   bloody flux, bowel movement, cachexia, cachexy, cardialgia,
   catharsis, chill, chills, cholera morbus, colic, constipation,
   convulsion, costiveness, coughing, crap, cyanosis, defecation,
   dejection, dizziness, dropsy, dysentery, dyspepsia, dyspnea, edema,
   emaciation, evacuation, fainting, fatigue, fever, fibrillation,
   flux, gripe, gripes, growth, heartburn, hemorrhage,
   high blood pressure, hydrops, hypertension, hypotension, icterus,
   indigestion, inflammation, insomnia, irregularity, itching,
   jaundice, labored breathing, lientery, loose bowels,
   low blood pressure, lumbago, marasmus, movement, nasal discharge,
   nausea, necrosis, obstipation, pain, paralysis, pruritus,
   purgation, purge, pyrosis, rash, rheum, runs, sclerosis, seizure,
   shit, shits, shock, skin eruption, sneezing, sore, spasm, stool,
   tabes, tachycardia, trots, tumor, turistas, upset stomach, vertigo,
   voidance, vomiting, wasting