The Collaborative International Dictionary of English v.0.48:

Avulsion \A*vul"sion\, n. [L. avulsio.]
   1. A tearing asunder; a forcible separation.
      [1913 Webster]

            The avulsion of two polished superficies. --Locke.
      [1913 Webster]

   2. A fragment torn off. --J. Barlow.
      [1913 Webster]

   3. (Law) The sudden removal of lands or soil from the estate
      of one man to that of another by an inundation or a
      current, or by a sudden change in the course of a river by
      which a part of the estate of one man is cut off and
      joined to the estate of another. The property in the part
      thus separated, or cut off, continues in the original
      owner. --Wharton. Burrill.
      [1913 Webster]

WordNet (r) 3.0 (2006):

avulsion
    n 1: an abrupt change in the course of a stream that forms the
         boundary between two parcels of land resulting in the loss
         of part of the land of one landowner and a consequent
         increase in the land of another
    2: a forcible tearing or surgical separation of one body part
       from another

Bouvier's Law Dictionary, Revised 6th Ed (1856):

AVULSION. Where, by the immediate and manifest power of a river or stream,
the soil is taken suddenly from one man's estate and carried to another. In
such case the property belongs to the first owner. An acquiescence on his
part, however, will in time entitle the owner of the land to which it is
attached to claim it as his own. Bract. 221; Harg. Tracts, De jure maris,
&c. Toull. Dr. Civ. Fr. tom. 3, p. 106; 2. Bl. Com. 262; Schultes on Aq.
Rights, 115 to 138. Avulsion differs from alluvion (q.v.) in this, that in
the latter case the change of the soil is gradual and imperceptible.