The Collaborative International Dictionary of English v.0.48:

Furlong \Fur"long\, n. [OE. furlong, furlang, AS. furlang,
   furlung, prop., the length of a furrow; furh furrow + lang
   long. See Furrow, and Long, a.]
   A measure of length; the eighth part of a mile; forty rods;
   two hundred and twenty yards.
   [1913 Webster]

WordNet (r) 3.0 (2006):

furlong
    n 1: a unit of length equal to 220 yards

Easton's 1897 Bible Dictionary:

Furlong
   a stadium, a Greek measure of distance equal to 606 feet and 9
   inches (Luke 24:13; John 6:19; 11:18; Rev. 14:20; 21:16).


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

FURLONG. A measure of length, being forty poles, or one-eighth of a mile.
Vide Measures.