The Collaborative International Dictionary of English v.0.48:

Post-obit \Post-o"bit\, n., or Post-obit bond \Post-o"bit bond`\
   . [Pref. post- + obit.] (Law)
   A bond in which the obligor, in consideration of having
   received a certain sum of money, binds himself to pay a
   larger sum, on unusual interest, on the death of some
   specified individual from whom he has expectations.
   --Bouvier.
   [1913 Webster]

WordNet (r) 3.0 (2006):

post-obit bond
    n 1: a bond made by a reversioner to secure a loan; payable out
         of his reversion