The Collaborative International Dictionary of English v.0.48:

Arpent \Ar"pent\, Arpen \Ar"pen\, n. [F. arpent, fr. L.
   arepennis, arapennis. According to Columella, a Gallic word
   for a measure equiv. to half a Roman jugerum.]
   Formerly, a measure of land in France, varying in different
   parts of the country. The arpent of Paris was 4,088 sq.
   yards, or nearly five sixths of an English acre. The woodland
   arpent was about 1 acre, 1 rood, 1 perch, English.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arpent
    n 1: a former French unit of area; equal approximately to an
         acre

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

ARPENT. A quantity of land containing a French acre. 4 Hall's Law Journal,
518.