The Collaborative International Dictionary of English v.0.48:

Horseback \Horse"back`\, n.
   1. The back of a horse.
      [1913 Webster]

   2. An extended ridge of sand, gravel, and boulders, in a
      half-stratified condition. --Agassiz.
      [1913 Webster]

   On horseback, on the back of a horse; mounted or riding on
      a horse or horses; in the saddle.
      [1913 Webster]

            The long journey was to be performed on horseback.
                                                  --Prescott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

horseback
    adv 1: on the back of a horse; "he rode horseback to town";
           "managed to escape ahorse"; "policeman patrolled the
           streets ahorseback" [syn: horseback, ahorse,
           ahorseback]
    n 1: the back of a horse
    2: a narrow ridge of hills [syn: hogback, horseback]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "horseback":
   across, astraddle, astride, by horse, chine, col, comb, esker,
   hogback, kame, mounted, on, on horseback, ridge, saddle,
   saddleback, spine, straddle, straddle-legged, straddleback