The Collaborative International Dictionary of English v.0.48:

Exheredation \Ex*her`e*da"tion\, n. [L., exheredatio: cf. F.
   exh['e]r['e]dation.]
   A disinheriting; disherison. [R.]

   Syn: exhereditation. [1913 Webster]

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

EXHEREDATION, civil law. The act by which a forced heir is deprived of his
legitimate or legal portion which the law gives him; disinherison. (q.v.)