The Collaborative International Dictionary of English v.0.48:

Ademption \A*demp"tion\ ([.a]*d[e^]mp"sh[u^]n), n. [L. ademptio,
   fr. adimere, ademptum, to take away; ad + emere to buy, orig.
   to take.] (Law)
   The revocation or taking away of a grant, donation, legacy,
   or the like. --Bouvier.
   [1913 Webster] Aden

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

ADEMPTION, wills. A taking away or revocation of a legacy, by the testator.
     2. It is either express or implied. It is the former when revoked in
express terms by a codicil or later will; it is implied when by the acts of
the testator it is manifestly his intention to revoke it; for example, when
a specific legacy of, a chattel is made, and afterwards the testator sells
it; or if a father makes provision for a child by his will and afterwards
gives to such child, if a daughter, a portion in marriage; or, if a son, a
sum of money to establish him in life, provided such portion or sum of money
be equal to or greater than the legacy. 2 Fonbl. 368 et, seq. Toll. Ex. 320;
1 Vern. R. by Raithby, 85 n. and the cases there cited. 1 Roper, Leg. 237,
256, for, the distinction between specific and general legacies.