The Collaborative International Dictionary of English v.0.48:

Severity \Se*ver"i*ty\, n.; pl. Severities. [L. severitas: cf.
   F. s['e]v['e]rit['e].]
   The quality or state of being severe. Specifically:
   [1913 Webster]
   (a) Gravity or austerity; extreme strictness; rigor;
       harshness; as, the severity of a reprimand or a reproof;
       severity of discipline or government; severity of
       penalties. "Strict age, and sour severity." --Milton.
       [1913 Webster]
   (b) The quality or power of distressing or paining; extreme
       degree; extremity; intensity; inclemency; as, the
       severity of pain or anguish; the severity of cold or
       heat; the severity of the winter.
       [1913 Webster]
   (c) Harshness; cruel treatment; sharpness of punishment; as,
       severity practiced on prisoners of war.
       [1913 Webster]
   (d) Exactness; rigorousness; strictness; as, the severity of
       a test.
       [1913 Webster]

             Confining myself to the severity of truth.
                                                  --Dryden.
       [1913 Webster]

WordNet (r) 3.0 (2006):

severity
    n 1: used of the degree of something undesirable e.g. pain or
         weather [syn: badness, severity, severeness]
    2: something hard to endure; "the asperity of northern winters"
       [syn: asperity, grimness, hardship, rigor, rigour,
       severity, severeness, rigorousness, rigourousness]
    3: extreme plainness [syn: austereness, severity,
       severeness]
    4: 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 "severity":
   Spartan simplicity, Spartanism, abruptness, absolute zero,
   absoluteness, abusiveness, accuracy, acerbity, acidity,
   acidulousness, acridity, acrimony, acuteness, aggressiveness,
   algidity, aloofness, animality, asceticism, asperity, astringency,
   atrocity, attention to fact, austerity, authoritarianism, baldness,
   barbarity, bareness, bearishness, beastliness, bite, bitingness,
   bitterness, bleakness, bloodlust, bluntness, boorishness,
   brashness, briskness, brusqueness, brusquerie, brutality,
   burdensomeness, candor, care for truth, causticity, chill,
   chilliness, churlishness, cold, coldness, common speech, cool,
   coolness, coolth, correctness, crispness, crudeness, cruelty,
   crustiness, cryogenics, cryology, curtness, cuttingness,
   decrease in temperature, delicacy, demandingness, despotism,
   destructiveness, directness, discipline, edge, elementarity,
   exactingness, exactitude, exactness, exigency, extremity,
   faithfulness, fastidiousness, faultlessness, ferociousness,
   ferocity, fidelity, fierceness, fineness, flawlessness, flintiness,
   force, frankness, freezing point, freshness, frigidity, frostiness,
   fundamentality, furiousness, fury, gelidity, gravity, grievousness,
   grimness, grip, gruffness, hardness, harshness, homespun,
   homogeneity, household words, iciness, impetuosity, inclemency,
   inexorability, inexorableness, inflexibility, inhumanity,
   inornateness, integrity, intense cold, intensity, keenness,
   leanness, literalism, literality, literalness, low temperature,
   malignity, mathematical precision, matter-of-factness, meanness,
   mercilessness, meticulousness, mindlessness, modesty, monasticism,
   monism, mordacity, mordancy, murderousness, nastiness, naturalness,
   niceness, nicety, nip, nippiness, obduracy, obdurateness, oneness,
   onerousness, openness, oppression, oppressiveness, painfulness,
   perfection, piquancy, pitilessness, plain English, plain speaking,
   plain speech, plain style, plain words, plainness, poignancy,
   point, preciseness, precision, primitiveness, prosaicness,
   prosiness, pungency, punishment, punitiveness, purity, rawness,
   refinement, regimentation, relentlessness, restrainedness, right,
   rightness, rigid discipline, rigidity, rigor, rigorousness,
   roughness, ruggedness, rustic style, ruthlessness, savagery,
   seriousness, sharp air, sharpness, shortness, simpleness,
   simplicity, singleness, soberness, solemnity, sourness, spareness,
   sparseness, starkness, sternness, sting, storminess,
   straightforwardness, strictness, stridency, stringency, subtlety,
   surliness, tartness, teeth, tempestuousness, terrorism, textualism,
   the letter, toughness, trenchancy, truculence, tyranny,
   unadorned style, unadornedness, unadulteration, unaffectedness,
   unelaborateness, unfanciness, unfussiness, ungentleness,
   uniformity, unimaginativeness, unity, unmixedness, unpoeticalness,
   unsophistication, vandalism, vehemence, venom, vernacular,
   viciousness, violence, virulence