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