The Collaborative International Dictionary of English v.0.48:

Hackamore \Hack"a*more\ (h[a^]k"[.a]*m[=o]r), n. [Cf. Sp.
   jaquima headstall of a halter.]
   A halter consisting of a long leather or rope strap and
   headstall, -- used for leading or tieing a pack animal.
   [Western U. S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

hackamore
    n 1: rope or canvas headgear for a horse, with a rope for
         leading [syn: halter, hackamore]