Definify.com
Webster 1913 Edition
Horseback
Horse′backˊ
,Noun.
1.
The back of a horse.
2.
An extended ridge of sand, gravel, and boulders, in a half-stratified condition.
Agassiz.
On horseback
, on the back of a horse; mounted or riding on a horse or horses; in the saddle.
The long journey was to be performed
on horseback
. Prescott.
Webster 1828 Edition
Horseback
HORSEBACK
,Noun.
I saw them salute on horseback.