The Collaborative International Dictionary of English v.0.48:

Complication \Com`pli*ca"tion\, n. [L. compliasion: cf. F.
   complication.]
   1. The act or process of complicating; the state of being
      complicated; intricate or confused relation of parts;
      entanglement; complexity.
      [1913 Webster]

            A complication of diseases.           --Macaulay.
      [1913 Webster]

            Through and beyond these dark complications of the
            present, the New England founders looked to the
            great necessities of future times.    --Palfrey.
      [1913 Webster]

   2. (Med.) A disease or diseases, or adventitious
      circumstances or conditions, coexistent with and modifying
      a primary disease, but not necessarily connected with it.
      [1913 Webster]

WordNet (r) 3.0 (2006):

complication
    n 1: the act or process of complicating
    2: a situation or condition that is complex or confused; "her
       coming was a serious complication"
    3: any disease or disorder that occurs during the course of (or
       because of) another disease; "bed sores are a common
       complication in cases of paralysis"
    4: a development that complicates a situation; "the court's
       decision had many unforeseen ramifications" [syn:
       complication, ramification]
    5: puzzling complexity [syn: complicatedness, complication,
       knottiness, tortuousness]

Moby Thesaurus II by Grady Ward, 1.0:

202 Moby Thesaurus words for "complication":
   abnormality, abstruseness, action, acute disease, affection,
   affliction, ailment, allergic disease, allergy, anagnorisis, angle,
   architectonics, architecture, arduousness, argument, atmosphere,
   atrophy, background, bacterial disease, bind, birth defect, blight,
   bothersomeness, burdensomeness, cardiovascular disease,
   catastrophe, characterization, chronic disease,
   circulatory disease, clutch, color, complaint, complexity,
   complexness, condition, congenital defect, continuity, contrivance,
   convolution, crabbedness, crampedness, crunch, deepness, defect,
   deficiency disease, deformity, degenerative disease, denouement,
   design, development, device, difficultness, difficulty, dilemma,
   disability, disease, disorder, distemper, drawback,
   embarrassing position, embarrassment, endemic, endemic disease,
   endocrine disease, entanglement, epidemic disease, episode,
   esoterica, fable, falling action, fine how-do-you-do,
   functional disease, fungus disease, gastrointestinal disease,
   genetic disease, gimmick, hairiness, handicap, hardness,
   hell to pay, hereditary disease, hobble, hot water, how-do-you-do,
   iatrogenic disease, illness, imbroglio, incident, indisposition,
   infectious disease, infirmity, intricacy, intricateness,
   involution, involvement, jam, knottiness, laboriousness, line,
   local color, malady, malaise, mess, mix, mood, morass, morbidity,
   morbus, motif, movement, muscular disease, mythos,
   neurological disease, nutritional disease, obstacle, obstruction,
   occupational disease, onerousness, oppressiveness, organic disease,
   pandemic disease, parlous straits, pass, pathological condition,
   pathology, peripeteia, perplexity, pickle, pinch, plan,
   plant disease, plight, plot, predicament, pretty pass,
   pretty pickle, pretty predicament, problem, profoundness,
   profundity, protozoan disease, psychosomatic disease, quagmire,
   quicksand, ramification, recognition, reconditeness,
   respiratory disease, rigor, rigorousness, rising action, rockiness,
   ruggedness, scheme, scrape, secondary disease, secondary plot,
   seediness, sickishness, sickness, signs, slant, slough, snag, spot,
   squeeze, stew, sticky wicket, story, strait, straits,
   strenuousness, structure, subject, subplot, subtlety, swamp,
   switch, symptomatology, symptomology, symptoms, syndrome,
   tanglement, technicality, the pip, thematic development, theme,
   tight spot, tight squeeze, tightrope, toilsomeness, tone, topic,
   toughness, tricky spot, troublesomeness, twist, unholy mess,
   urogenital disease, virus disease, wasting disease, worm disease