The Collaborative International Dictionary of English v.0.48:

Austerity \Aus*ter"i*ty\, n.; pl. Austerities. [F.
   aust['e]rit['e], L. austerias, fr. austerus. See Austere.]
   1. Sourness and harshness to the taste. [Obs.] --Horsley.
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   2. Severity of manners or life; extreme rigor or strictness;
      harsh discipline.
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            The austerity of John the Baptist.    --Milton.
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   3. Plainness; freedom from adornment; severe simplicity.
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            Partly owing to the studied austerity of her dress,
            and partly to the lack of demonstration in her
            manners.                              --Hawthorne.
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WordNet (r) 3.0 (2006):

austerity
    n 1: the trait of great self-denial (especially refraining from
         worldly pleasures) [syn: austerity, asceticism,
         nonindulgence]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "austerity":
   Albigensianism, Catharism, Franciscanism, Lenten fare,
   Sabbatarianism, Spartan simplicity, Spartanism, Trappism,
   Waldensianism, Yoga, abstinence, anchoritic monasticism,
   anchoritism, asceticism, astringency, austerity program,
   authoritarianism, baldness, bareness, candor, canniness, care,
   carefulness, chariness, common speech, demandingness, directness,
   discipline, economic planning, economicalness, economy,
   economy of means, eremitism, exactingness, exiguity, exiguousness,
   false economy, fasting, flagellation, forehandedness, frankness,
   frugality, frugalness, good management, grimness, hardness,
   harshness, homespun, household words, husbandry, inclemency,
   inornateness, jejuneness, jejunity, leanness, maceration,
   management, matter-of-factness, meagerness, meanness, mendicantism,
   meticulousness, miserliness, monachism, monasticism, mortification,
   narrowness, naturalness, niggardliness, openness, paltriness,
   parsimoniousness, parsimony, plain English, plain speaking,
   plain speech, plain style, plain words, plainness, prosaicness,
   prosiness, providence, prudence, prudential administration,
   puniness, puritanism, regimentation, restrainedness,
   rigid discipline, rigor, roughness, ruggedness, rustic style,
   scantiness, scantness, scrawniness, scrimpiness, self-denial,
   self-mortification, severity, simpleness, simplicity, skimpiness,
   slenderness, slightness, slim pickings, slimness, smallness,
   soberness, spareness, sparingness, starkness, sternness,
   stinginess, straightforwardness, strictness, stringency, thinness,
   thrift, thriftiness, tight purse strings, toughness,
   unadorned style, unadornedness, unaffectedness, unelaborateness,
   unfanciness, unfussiness, ungentleness, unimaginativeness,
   unpoeticalness, unwastefulness, vernacular, voluntary poverty