Source : Webster's Revised Unabridged Dictionary (1913)

Reddendum \Red*den"dum\ (r?d*d?n"d?m), n. [Neut. of L. reddendus
   that must be given back or yielded, gerundive of reddere. See
   {Reddition}.] (Law)
   A clause in a deed by which some new thing is reserved out of
   what had been granted before; the clause by which rent is
   reserved in a lease. --Cruise.