The Collaborative International Dictionary of English v.0.48:

Allodium \Al*lo"di*um\, n. [LL. allodium, alodium, alodis,
   alaudis, of Ger. origin; cf. OHG. al all, and ?t (AS. e[=a]d)
   possession, property. It means, therefore, entirely one's
   property.] (Law)
   Freehold estate; land which is the absolute property of the
   owner; real estate held in absolute independence, without
   being subject to any rent, service, or acknowledgment to a
   superior. It is thus opposed to feud. --Blackstone.
   --Bouvier.
   [1913 Webster]

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

ALLODIUM estates. Signifies an absolute estate of inheritance, in
contradistinction to a feud.
     2. In this country the title to land is essentially allodial, and every
tenant in fee simple has an absolute and perfect title, yet in technical
language his estate is called an estate in fee simple, and the tenure free
and common socage. 3 Kent, Com. 390; Cruise, Prel. Dis. c. 1, Sec. 13; 2 Bl.
Com. 45. For the etymology of this word, vide 3 Kent Com. 398 note; 2 Bouv.
Inst. n. 1692.