The Collaborative International Dictionary of English v.0.48:

Lariat \Lar"i*at\ (l[a^]r"[i^]*[a^]t), n. [Sp. la reata the
   rope; la the + reata rope. Cf. Reata.]
   A long, slender rope made of hemp or strips of hide, esp. one
   with a noose; -- used as a lasso for catching cattle, horses,
   etc., and for picketing a horse so that he can graze without
   wandering. [Mexico & Western U.S.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lariat \Lar"i*at\ (l[a^]r"[i^]*[a^]t), v. t. [imp. & p. p.
   Lariated; p. pr. & vb. n. Lariating.]
   To secure with a lariat fastened to a stake, as a horse or
   mule for grazing; also, to lasso or catch with a lariat.
   [Western U.S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

lariat
    n 1: a long noosed rope used to catch animals [syn: lasso,
         lariat, riata, reata]