The Collaborative International Dictionary of English v.0.48:

Ablation \Ab*la"tion\, n. [L. ablatio, fr. ablatus p. p. of
   auferre to carry away; ab + latus, p. p. of ferre carry: cf.
   F. ablation. See Tolerate.]
   1. A carrying or taking away; removal. --Jer. Taylor.
      [1913 Webster]

   2. (Med.) Extirpation. --Dunglison. AS
      [1913 Webster]

   3. (Med.) Removing or destroying of a body tissue, especially
      by a surgical procedure. --Dorland.
      [AS]

   4. (Geol.) Wearing away; superficial waste, as of glacial ice
      or snow. --Tyndall.
      [1913 Webster]

   5. (Aerospace) Wearing away of the outer layers of a
      protective shield or surface by the heat and aerodynamic
      forces caused by flying through the atmosphere at
      hypersonic speed, as during reentry from space; as,
      ablation of the heat shield during reentry.
      [PJC]

WordNet (r) 3.0 (2006):

ablation
    n 1: surgical removal of a body part or tissue [syn: ablation,
         extirpation, cutting out, excision]
    2: the erosive process that reduces the size of glaciers