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]