The Collaborative International Dictionary of English v.0.48:

Rigor \Ri"gor\, n. [L. See Rigor., below.]
   1. Rigidity; stiffness.
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   2. (ed.) A sense of chilliness, with contraction of the skin;
      a convulsive shuddering or tremor, as in the chill
      preceding a fever.
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   Rigor caloris[L., rigor of heat] (Physiol.), a form of
      rigor mortis induced by heat, as when the muscle of a
      mammal is heated to about 50[deg] C.

   Rigor mortis[L., rigor of death], death stiffening; the
      rigidity of the muscles that occurs at death and lasts
      till decomposition sets in. It is due to the formation of
      myosin by the coagulation of the contents of the
      individual muscle fibers.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rigor \Rig"or\, n. [OE. rigour, OF. rigour, F. rigueur, from L.
   rigor, fr. rigere to be stiff. See Rigid.] [Written also
   rigour.]
   1. The becoming stiff or rigid; the state of being rigid;
      rigidity; stiffness; hardness.
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            The rest his look
            Bound with Gorgonian rigor not to move. --Milton.
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   2. (Med.) See 1st Rigor, 2.
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   3. Severity of climate or season; inclemency; as, the rigor
      of the storm; the rigors of winter.
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   4. Stiffness of opinion or temper; rugged sternness;
      hardness; relentless severity; hard-heartedness; cruelty.
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            All his rigor is turned to grief and pity. --Denham.
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            If I shall be condemn'd
            Upon surmises, . . . I tell you
            'T is rigor and not law.              --Shak.
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   5. Exactness without allowance, deviation, or indulgence;
      strictness; as, the rigor of criticism; to execute a law
      with rigor; to enforce moral duties with rigor; -- opposed
      to lenity.
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   6. Severity of life; austerity; voluntary submission to pain,
      abstinence, or mortification.
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            The prince lived in this convent with all the rigor
            and austerity of a capuchin.          --Addison.
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   7. Violence; force; fury. [Obs.]
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            Whose raging rigor neither steel nor brass could
            stay.                                 --Spenser.
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   Syn: Stiffness; rigidness; inflexibility; severity;
        austerity; sternness; harshness; strictness; exactness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

rigor
    n 1: something hard to endure; "the asperity of northern
         winters" [syn: asperity, grimness, hardship, rigor,
         rigour, severity, severeness, rigorousness,
         rigourousness]
    2: the quality of being valid and rigorous [syn: cogency,
       validity, rigor, rigour]
    3: excessive sternness; "severity of character"; "the harshness
       of his punishment was inhuman"; "the rigors of boot camp"
       [syn: severity, severeness, harshness, rigor,
       rigour, rigorousness, rigourousness, inclemency,
       hardness, stiffness]

Moby Thesaurus II by Grady Ward, 1.0:

240 Moby Thesaurus words for "rigor":
   Albigensianism, Catharism, Franciscanism, Sabbatarianism, Trappism,
   Waldensianism, Yoga, absolute zero, absoluteness, abstinence,
   abstruseness, accuracy, acerbity, acidity, acridity, acrimony,
   acuteness, adverse circumstances, adversity, affliction,
   aggravation, algidity, anchoritic monasticism, anchoritism,
   animality, annoyance, arduousness, asceticism, asperity,
   astringency, atrocity, attention to detail, attention to fact,
   austerity, barbarity, bite, bitterness, bleakness, blight,
   bloodlust, bothersomeness, briskness, brutality, bummer,
   burdensomeness, care, care for truth, causticity, chill,
   chilliness, circumstantiality, cold, coldness, complexity,
   complication, conscientiousness, cool, coolness, coolth,
   correctness, crispness, criticality, criticalness, cross,
   cryogenics, cryology, curse, decrease in temperature, delicacy,
   destructiveness, detail, difficulties, difficultness, difficulty,
   downer, edge, eremitism, exactingness, exactitude, exactness,
   exquisiteness, extremity, faithfulness, fasting, faultlessness,
   ferociousness, fidelity, fierceness, fineness, finicality,
   finicalness, finickiness, finickingness, firmness, flagellation,
   flawlessness, force, freezing point, freshness, frigidity,
   frostiness, fundamentalism, furiousness, fussiness, gelidity, grip,
   hairiness, hard knocks, hard life, hard lot, hardcase, hardness,
   hardship, harshness, iciness, impetuosity, impliability,
   inclemency, inexorability, inflexibility, inhospitableness,
   inhumanity, intense cold, intensity, intricacy, irritation,
   keenness, laboriousness, literalism, literality, literalness,
   low temperature, maceration, malignity, mathematical precision,
   mendicantism, mercilessness, meticulousness, mindlessness,
   minuteness, monachism, monasticism, mordacity, mordancy,
   mortification, murderousness, niceness, nicety, nip, nippiness,
   obduracy, obdurateness, obstinacy, onerousness, oppressiveness,
   orthodoxy, particularity, particularness, perfection, pitilessness,
   plight, poignancy, point, preciseness, precisianism, precision,
   predicament, pressure, punctilio, punctiliousness, punctuality,
   purism, puritanism, rawness, refinement, relentlessness, renitence,
   renitency, right, rightness, rigidity, rigidness, rigorism,
   rigorousness, roughness, ruggedness, savagery, scrupulosity,
   scrupulousness, sea of troubles, self-denial, self-mortification,
   severity, sharp air, sharpness, starchiness, sternness, stiffness,
   sting, strenuousness, stress, stress of life, strictness,
   stridency, stringency, stubbornness, subtlety, tartness, tautness,
   teeth, tenseness, tension, tensity, terrorism, textualism,
   the letter, tightness, toilsomeness, toughness, trenchancy, trial,
   tribulation, trouble, troubles, troublesomeness, unbendingness,
   uncompromisingness, ungentleness, unrelentingness, unyieldingness,
   vale of tears, vandalism, vehemence, venom, viciousness,
   vicissitude, violence, virulence, visitation, voluntary poverty