The Collaborative International Dictionary of English v.0.48:

Erosion \E*ro"sion\, n. [L. erosio. See Erode.]
   1. The act or operation of eroding or eating away.
      [1913 Webster]

   2. The state of being eaten away; corrosion; canker.
      [1913 Webster]

   3. The wearing away of the earth's surface by any natural
      process. The chief agent of erosion is running water;
      minor agents are glaciers, the wind, and waves breaking
      against the coast.
      [Webster 1913 Suppl.]

   4. a gradual reduction or lessening as if by an erosive
      force; as, erosion of political support due to scandal;
      erosion of buying power by inflation. [fig.]
      [PJC]

WordNet (r) 3.0 (2006):

erosion
    n 1: (geology) the mechanical process of wearing or grinding
         something down (as by particles washing over it) [syn:
         erosion, eroding, eating away, wearing, wearing
         away]
    2: condition in which the earth's surface is worn away by the
       action of water and wind
    3: a gradual decline of something; "after the accounting scandal
       there was an erosion of confidence in the auditors"
    4: erosion by chemical action [syn: corrosion, corroding,
       erosion]

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "erosion":
   ablation, abrading, abrasion, abrasive, absorption, abstraction,
   assimilation, atomization, attrition, breakup, buffing, burning up,
   burnishing, chafe, chafing, consumption, corroding, corrosion,
   crumbling, decay, decomposition, decrease, decrement, deduction,
   degradation, deliquescence, depletion, depreciation, detrition,
   digestion, dilapidation, disintegration, disjunction,
   disorganization, dissipation, dissolution, drain, dressing,
   eating up, erasure, evaporation, exhaustion, expending,
   expenditure, filing, finishing, fraying, fretting, galling,
   grazing, grinding, impoverishment, incoherence, ingestion, leakage,
   limation, loss, polishing, purification, rasping, ravages of time,
   refinement, removal, resolution, rubbing away, sandblasting,
   sanding, scouring, scrape, scraping, scratch, scratching, scrub,
   scrubbing, scuff, shining, shrinkage, smoothing, spending,
   squandering, subduction, sublation, subtraction, taking away, use,
   using, using up, wastage, waste, wastefulness, wasting away, wear,
   wear and tear, wearing, wearing away, wearing down, weathering