The phrase "horizontal orientation" refers to the alignment or positioning of something in a horizontal or side-to-side direction, rather than vertical or up-and-down. Full definition
For instance, they fixed a bug that would cause the screen to go black when exiting a game in horizontal orientation. (xda-developers.com)
Perhaps, they thought it made more sense with the more common horizontal orientation of tablets, but it would have been nice if it were a customizable option of some sort. (theverge.com)
Both nevertheless elevate Held's awareness of objective and subjective differences in terms of identical formats (both works are 24 by 32 inches, one with horizontal orientation, the other vertical). (brooklynrail.org)