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.
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WordNet (r) 3.0 (2006):

rigour
    n 1: the quality of being valid and rigorous [syn: cogency,
         validity, rigor, rigour]
    2: something hard to endure; "the asperity of northern winters"
       [syn: asperity, grimness, hardship, rigor, rigour,
       severity, severeness, rigorousness, rigourousness]
    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]