The Collaborative International Dictionary of English v.0.48:

Registrant \Reg"is*trant\ (-trant), n. [L. registrans, p. pr.]
   One who registers; esp., one who, by virtue of securing an
   official registration, obtains a certain right or title of
   possession, as to a trade-mark.
   [1913 Webster]

WordNet (r) 3.0 (2006):

registrant
    n 1: a person who is formally entered (along with others) in a
         register (and who obtains certain rights thereby)