The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
reversioner n 1: (law) a party who is entitled to an estate in reversion