The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

REDDENDUM, contracts. A word used substantively, and is that clause in a
deed by which the grantor reserves something new to himself out of that
which he granted before, and thus usually follows the tenendum, and is
generally in these words "yielding and paying."
     2. In every good reddendum or reservation, these things must concur;
namely,  1. It must be apt words. 2, It must be of some other thing issuing
or coming out of the thing granted, and not a part of the thing itself, nor
of something issuing out of another thing. 3. It must be of such thing on
which the grantor may resort to distrain 4. It must be made to one of the
grantors and not to a stranger to the deed. Vid 2 Bl. Com. 299; Co. Litt.
47; Touchs 80; Cruise, Dig. tit. 32, c. 24, s. 1; Dane' Ab. Index, h.t.