The Collaborative International Dictionary of English v.0.48:

Exhaustion \Ex*haus"tion\, n. [Cf. F. exhaustion.]
   1. The act of draining out or draining off; the act of
      emptying completely of the contents.
      [1913 Webster]

   2. The state of being exhausted or emptied; the state of
      being deprived of strength or spirits.
      [1913 Webster]

   3. (Math.) An ancient geometrical method in which an
      exhaustive process was employed. It was nearly equivalent
      to the modern method of limits.
      [1913 Webster]

   Note: The method of exhaustions was applied to great variety
         of propositions, pertaining to rectifications and
         quadratures, now investigated by the calculus.
         [1913 Webster]

WordNet (r) 3.0 (2006):

exhaustion
    n 1: extreme fatigue
    2: serious weakening and loss of energy [syn: debilitation,
       enervation, enfeeblement, exhaustion]
    3: the act of exhausting something entirely

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "exhaustion":
   abatement, ablation, absorption, assimilation, attenuation,
   attrition, blunting, breakdown, burning up, cachexia, cachexy,
   chronic ill health, circulatory collapse, cleaning out, clearance,
   clearing, collapse, consumption, corrosion, crack-up, crackup,
   damping, deadening, debilitation, debility, decrease, decrement,
   decrepitude, defecation, delicacy, delicate health, deliquescence,
   depletion, depreciation, devitalization, digestion, dilution,
   discharge, discharging cargo, dissipation, dissolution, drain,
   drainage, draining, drawing out, dulling, eating up, effemination,
   egress, elimination, emptying, enervation, enfeeblement, erosion,
   evacuation, evaporation, evisceration, excretion, exhaustedness,
   exhausting, expending, expenditure, extenuation, fatigue,
   feebleness, finishing, fragility, frailty, healthlessness,
   hypochondria, hypochondriasis, ill health, impoverishment,
   inanition, infirmity, ingestion, invalidism, invalidity,
   languishing, languishment, lassitude, leakage, loss, mitigation,
   morbidity, morbidness, nervous breakdown, nervous exhaustion,
   nervous prostration, neurasthenia, off-loading, peakedness,
   poor health, prostration, reduction, relaxation, removal,
   shrinkage, sickliness, slackening, softening, spending,
   squandering, thinning, tiredness, unhealthiness, unloading,
   unsoundness, unwholesomeness, using, using up, valetudinarianism,
   venting, voidance, voiding, wastage, waste, wastefulness, wasting,
   wasting away, weakening, weakliness, wear and tear, weariness,
   wearing, wearing away, wearing down