Definitions:
a horizontal plane or line with respect to the distance above or below a given point

a position on a scale of amount, quantity, extent, or quality

(in a video game) each of a series of stages of increasing difficulty through which a player may progress, completing one stage in order to reach the next

an instrument marked with a line parallel to the plane of the horizon for testing whether things are horizontal.

a flat tract of land

having a flat, horizontal surface

at the same height as someone or something else

having the same relative position; not in front of or behind

calm and steady

give a flat and even surface to

make (something, especially a score in sport) equal or similar

aim (a weapon)

ascertain differences in the height of (land).