The Collaborative International Dictionary of English v.0.48:

Fief \Fief\, n. [F. fief; of German origin, and the same word as
   E. fee. See Fee, and cf. Feud, a tief.] (Law)
   An estate held of a superior on condition of military
   service; a fee; a feud. See under Benefice, n., 2.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fief
    n 1: a piece of land held under the feudal system [syn: fief,
         feoff]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "fief":
   copyhold, equitable estate, estate at sufferance, estate for life,
   estate for years, estate in expectancy, estate in fee,
   estate in possession, estate tail, fee, fee simple, fee tail, feod,
   feodum, feud, feudal estate, lease, leasehold, legal estate,
   paramount estate, particular estate, remainder, reversion,
   vested estate



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

FIEF, or FEUD. In its origin, a fief was a district of country allotted to
one of the chiefs who invaded the Roman empire, as a stipend or reward; with
a condition annexed that the possessor should do service faithfully both at
home and in the wars, to him by whom it was given. The law of fiefs supposed
that originally all lands belonged to lords, who had had the generosity to
abandon them to others, from whom the actual possessors derive their rights
upon the sole reservation of certain services more or less onerous as a sign
of superiority. To this superiority was added that which gives the right of
dispensing justice, a right which was originally attached to all fiefs, and
conferred upon those who possessed it, the most eminent part of public
power. Henrion de Pansey, Pouvoir, Municipal; 2 Bl. Com. 45
Encyclopedie, h.t.; Merl. Rep. h.t.