Source : Webster's Revised Unabridged Dictionary (1913)

Horizontally \Hor`i*zon"tal*ly\, adv.
   In a horizontal direction or position; on a level; as, moving
   horizontally.

Source : WordNet®

horizontally
     adv : in a horizontal direction; "a gallery quite often is added
           to make use of space vertically as well as
           horizontally"