horizontally
adv.in a flat position, like the line where the sky meets the sea. You use this to describe things that go from left to right rather than up and down.
adv.parallel to the plane of the horizon; at right angles to the vertical. Often describes the orientation of objects or the direction of movement across a surface.
“The stripes on his shirt run horizontally.”
“To save space in the small apartment, we stacked the books vertically rather than laying them horizontally on the shelf.”
“The data is arranged horizontally across the spreadsheet, with each row representing a unique participant and each column representing a specific variable in the study.”
Typically follows the verb it modifies or appears at the end of the clause.
“UOG uses a process known as fracking to extract fuel from the ground, shooting a high-pressure mixture of water, sand and chemicals to break apart rock horizontally from the initial borehole.”
From horizontal + -ly.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.