The Collaborative International Dictionary of English v.0.48:

Syndrome \Syn"dro*me\, n. [NL., from Gr. ?; sy`n with + ? a
   course, a running.]
   1. Concurrence. [R.] --Glanvill.
      [1913 Webster]

   2. (Med.) A group of symptoms occurring together that are
      characteristic and indicative of some underlying cause,
      such as a disease.
      [1913 Webster]

   3. generally, a pattern of characteristics or behaviors
      occurring together, that are sufficiently common and
      distinctive to indicate that they are due to a known
      cause.
      [PJC]

WordNet (r) 3.0 (2006):

syndrome
    n 1: a complex of concurrent things; "every word has a syndrome
         of meanings"
    2: a pattern of symptoms indicative of some disease

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "syndrome":
   abnormality, acute disease, affection, affliction, ailment,
   allergic disease, allergy, atrophy, bacterial disease,
   birth defect, blight, cardiovascular disease, chronic disease,
   circulatory disease, complaint, complication, condition,
   congenital defect, defect, deficiency disease, deformity,
   degenerative disease, disability, disease, disorder, distemper,
   endemic, endemic disease, endocrine disease, epidemic disease,
   functional disease, fungus disease, gastrointestinal disease,
   genetic disease, handicap, hereditary disease, iatrogenic disease,
   ill, illness, indisposition, infectious disease, infirmity, malady,
   malaise, morbidity, morbus, muscular disease, neurological disease,
   nutritional disease, occupational disease, organic disease,
   pandemic disease, pathological condition, pathology, plant disease,
   protozoan disease, psychosomatic disease, respiratory disease,
   rockiness, secondary disease, seediness, sickishness, sickness,
   signs, symptomatology, symptomology, symptoms, the pip,
   urogenital disease, virus disease, wasting disease, worm disease