Source : Webster's Revised Unabridged Dictionary (1913)

Reversioner \Re*ver"sion*er\, n. (Law)
   One who has a reversion, or who is entitled to lands or
   tenements, after a particular estate granted is terminated.
   --Blackstone.

Source : WordNet®

reversioner
     n : (law) a party who is entitled to an estate in reversion