The Collaborative International Dictionary of English v.0.48:

Abrasion \Ab*ra"sion\, n. [L. abrasio, fr. abradere. See
   Abrade.]
   1. The act of abrading, wearing, or rubbing off; the wearing
      away by friction; as, the abrasion of coins.
      [1913 Webster]

   2. The substance rubbed off. --Berkeley.
      [1913 Webster]

   3. (Med.)
      (a) A superficial excoriation of skin or mucous membranes.
          --AS
      (b) Erosion of the tooth substance --AS
          [1913 Webster]

WordNet (r) 3.0 (2006):

abrasion
    n 1: an abraded area where the skin is torn or worn off [syn:
         abrasion, scratch, scrape, excoriation]
    2: erosion by friction [syn: abrasion, attrition,
       corrasion, detrition]
    3: the wearing down of rock particles by friction due to water
       or wind or ice [syn: grinding, abrasion, attrition,
       detrition]