The Collaborative International Dictionary of English v.0.48:

Ascetic \As*cet"ic\a. [Gr. ?, fr. ? to exercise, to practice
   gymnastics.]
   Extremely rigid in self-denial and devotions; austere;
   severe.
   [1913 Webster]

         The stern ascetic rigor of the Temple discipline. --Sir
                                                  W. Scott.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ascetic \As*cet"ic\, n.
   In the early church, one who devoted himself to a solitary
   and contemplative life, characterized by devotion, extreme
   self-denial, and self-mortification; a hermit; a recluse;
   hence, one who practices extreme rigor and self-denial in
   religious things.
   [1913 Webster]

         I am far from commending those ascetics that take up
         their quarters in deserts.               --Norris.
   [1913 Webster]

   Ascetic theology, the science which treats of the practice
      of the theological and moral virtues, and the counsels of
      perfection. --Am. Cyc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ascetic
    adj 1: pertaining to or characteristic of an ascetic or the
           practice of rigorous self-discipline; "ascetic practices"
           [syn: ascetic, ascetical]
    2: 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]
    n 1: someone who practices self denial as a spiritual discipline
         [syn: abstainer, ascetic]

Moby Thesaurus II by Grady Ward, 1.0:

221 Moby Thesaurus words for "ascetic":
   Albigensian, Apostolic, Apostolici, Catharist, Diogenes, Encratic,
   Encratite, Franciscan, Hieronymian, Hieronymite, Lenten,
   Pythagorean, Pythagorist, Rechabite, Sabbatarian, Shaker, Spartan,
   Stoic, Timon of Athens, Trappist, Waldensian, abbacomes, abbot,
   abstainer, abstemious, abstinent, anchoress, anchorite, anchoritic,
   apologetic, astringent, atoning, austere, bald, banian, bare,
   beadsman, bedridden invalid, bhikshu, brother, caloyer, candid,
   celibate, cenobite, chaste, cleansing, cloistered monk,
   closet cynic, common, commonplace, compensational, compensatory,
   continent, conventual, conventual prior, dervish, desert fathers,
   desert saints, direct, disciplined, dry, dull, dwarfed, dwarfish,
   eremite, eremitic, exiguous, expiatory, fakir, flagellant,
   forbearing, frank, friar, frugal, fruitarian, grand prior,
   gymnosophist, hermit, hermitess, hieromonach, homebody, homely,
   homespun, hydropot, impoverished, invalid, isolationist, jejune,
   lay abbot, lay brother, lean, limited, loner, lustral,
   lustrational, lustrative, marabout, matter-of-fact, meager, mean,
   mendicant, miserly, monastic, monk, mortified, narrow, natural,
   neat, nephalist, nephalistic, niggardly, on the wagon, open,
   outcast, palmer, paltry, pariah, parsimonious, penitential,
   piacular, pilgrim, pillar saint, pillarist, plain, plain-speaking,
   plain-spoken, poor, prior, propitiatory, prosaic, prosing, prosy,
   puny, pure, purgative, purgatorial, purifying, puritan,
   puritanical, reclamatory, recluse, recompensing, redeeming,
   redemptive, redressing, religieux, religious, reparative,
   reparatory, repentant, repenting, restitutional, restitutive,
   restitutory, restrained, righting, rigoristic, rustic, sannyasi,
   satisfactional, scant, scanty, schooled, scrawny, scrimp, scrimpy,
   seclusionist, self-abasing, self-abnegating, self-denying,
   self-forgetful, selfless, severe, sexually abstinent, shut-in,
   simple, simple-speaking, skimp, skimpy, slender, slight, slim,
   small, sober, solitaire, solitary, solitudinarian, spare, sparing,
   squaring, stark, starvation, stay-at-home, stern, stingy, stinted,
   straightforward, straitened, stunted, stylite, subsistence,
   sworn off, teetotal, teetotaler, teetotalist, thin, trained,
   unadorned, unaffected, unimaginative, unnourishing, unnutritious,
   unpoetical, unvarnished, vegetarian, water-drinker, watered,
   watery, wedded to poverty, yogi, yogin