"Horizon" refers to the line where the earth's surface and the sky appear to meet. It is a visual boundary that changes based on the observer's elevation and surroundings. The horizon can be seen clearly in open areas, such as fields or oceans, where there are few obstructions. In astronomy, the term "horizon" is also used to describe the event horizon of a black hole, which is the point beyond which nothing can escape its gravitational pull. This concept is crucial in understanding the nature of black holes and the limits of observable space.