The Collaborative International Dictionary of English v.0.48:

Austere \Aus*tere"\, [F. aust[`e]re, L. austerus, fr. Gr. ?, fr.
   ? to parch, dry. Cf. Sear.]
   1. Sour and astringent; rough to the state; having acerbity;
      as, an austere crab apple; austere wine.
      [1913 Webster]

   2. Severe in modes of judging, or living, or acting; rigid;
      rigorous; stern; as, an austere man, look, life.
      [1913 Webster]

            From whom the austere Etrurian virtue rose.
                                                  --Dryden.
      [1913 Webster]

   3. Unadorned; unembellished; severely simple.
      [1913 Webster]

   Syn: Harsh; sour; rough; rigid; stern; severe; rigorous;
        strict.
        [1913 Webster]

WordNet (r) 3.0 (2006):

austere
    adj 1: severely simple; "a stark interior" [syn: austere,
           severe, stark, stern]
    2: of a stern or strict bearing or demeanor; forbidding in
       aspect; "an austere expression"; "a stern face" [syn:
       austere, stern]
    3: practicing great self-denial; "Be systematically
       ascetic...do...something for no other reason than that you
       would rather not do it"- William James; "a desert nomad's
       austere life"; "a spartan diet"; "a spartan existence" [syn:
       ascetic, ascetical, austere, spartan]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "austere":
   Albigensian, Catharist, Franciscan, Lenten, Sabbatarian, Spartan,
   Spartanic, Trappist, Waldensian, abstemious, abstinent, amaroidal,
   anchoritic, ascetic, astringent, authoritarian, bald, bare, basic,
   biting, bitter, bleak, candid, chaste, common, commonplace,
   demanding, direct, dour, dry, dull, dwarfed, dwarfish, earnest,
   elementary, eremitic, essential, exacting, exigent, exiguous,
   flagellant, frank, frugal, fundamental, grave, grim, hard, harsh,
   homely, homespun, homogeneous, impoverished, inclement,
   indivisible, inornate, irreducible, jejune, keen, lean, limited,
   matter-of-fact, meager, mean, mendicant, mere, meticulous, miserly,
   monolithic, mortified, narrow, natural, neat, niggardly,
   of a piece, open, paltry, parsimonious, plain, plain-speaking,
   plain-spoken, poor, primal, primary, prosaic, prosing, prosy, puny,
   pure, pure and simple, puritanical, rigoristic, rough, rugged,
   rustic, scant, scanty, scrawny, scrimp, scrimpy, self-denying,
   serious, severe, sharp, simon-pure, simple, simple-speaking,
   single, skimp, skimpy, slender, slight, slim, small, sober, somber,
   spare, sparing, stark, starvation, stern, stingy, stinted,
   straightforward, straitened, strict, stringent, stunted,
   subsistence, thin, tough, unadorned, unaffected, uncluttered,
   undecorated, undifferenced, undifferentiated, unelaborate,
   unembellished, unfancy, unfussy, ungentle, uniform, unimaginative,
   unnourishing, unnutritious, unornamented, unornate, unpoetical,
   unsparing, unvarnished, watered, watery, wedded to poverty