the line at which the sky and Earth appear to meet
"As I walked along the beach, I watched the sun dip below the horizon where the blue sky meets the dark ocean."
a specific layer or stratum of soil or subsoil in a vertical cross section of land
"The geologist carefully mapped each horizon in the soil profile to determine the age of the sediment layers."
the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth
"The astronomer adjusted his instruments to measure the altitude of stars relative to the mathematical horizon defined by the plane passing through the observer's eye and the center of the Earth."
The visible horizontal line or point (in all directions) that appears to connect the Earth to the sky.
"As I climbed the hill, the distant horizon seemed to stretch endlessly where the green fields met the blue sky."
In plain English: The horizon is the line where the sky and ground seem to meet from your point of view.
"The ship disappeared over the horizon at sunset."
Usage: Use horizon to describe the apparent boundary where the earth meets the sky, rather than using it metaphorically for future possibilities unless specifically referring to one's personal outlook. This word is often confused with "horizonte" in Spanish contexts or misused as a synonym for "scope," but its primary meaning remains strictly visual and geographical.
The word "horizon" traveled into English through Middle and Modern French as a loanword. It ultimately derives from the Ancient Greek horízōn, which means "boundary."