der Horizont
horizon
The visible line where the sky and the earth or sea seem to meet.
Die Sonne versinkt langsam am Horizont.
The sun is slowly sinking on the horizon.

horizon
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horizon
The visible line where the sky and the earth or sea seem to meet.
Die Sonne versinkt langsam am Horizont.
The sun is slowly sinking on the horizon.

The adjective 'horizontal' is derived directly from 'Horizont' (horizon). It describes something that runs parallel to the horizon, meaning it is level or horizontal.
Bitte ziehe eine horizontale Linie von links nach rechts.
Please draw a horizontal line from left to right.
To describe a position at the horizon line, the preposition 'am' is almost always used. This is a fixed combination when talking about the sun, the moon, or ships.
Wir sahen ein kleines Segelboot am Horizont.
We saw a small sailboat on the horizon.
The most common figurative use is the fixed expression 'seinen Horizont erweitern' (to broaden one's horizon). It means to increase one's knowledge and experiences, for example through travel, reading, or learning new skills.
Ich möchte eine neue Sprache lernen, um meinen Horizont zu erweitern.
I want to learn a new language to broaden my horizon.
Besides 'erweitern' (to broaden/expand), 'Horizont' (horizon) is often used figuratively with 'einen weiten/engen Horizont haben' (to have a broad/narrow horizon). This describes a person's mental openness or limitations.
Er ist ein sehr gebildeter Mensch und hat einen weiten Horizont.
He is a very educated person and has a broad horizon.
In geology or archaeology, 'Horizont' (horizon) is a technical term for a particular rock or soil layer. You will find this meaning almost exclusively in scientific texts.
In diesem Horizont wurden besonders viele Fossilien gefunden.
A particularly large number of fossils were found in this horizon.
perspective / prospects
'die Perspektive' and 'der Horizont' can both have to do with the way someone sees the world. 'Perspektive' is a particular point of view on a topic or situation. 'Horizont', in its figurative sense, means the range of a person's knowledge, experience, and interests. So you can change your perspective when you look at something differently. You broaden your horizon when you learn new things or have new experiences. The risk of confusion comes from the fact that both words are connected with thinking and perception.

Reisen erweitert den Horizont, weil man andere Lebensweisen kennenlernt.
Travel broadens your horizon because you get to know other ways of life.
Im Gespräch mit meiner Freundin habe ich eine andere Perspektive auf das Problem bekommen.
In my conversation with my friend, I gained a different perspective on the problem.