The horizon is the line where the earth and sky appear to meet. When you stand on a beach and look out at the ocean, that distant line where the water seems to touch the sky is the horizon. On a flat prairie, the horizon circles all around you, marking the farthest point you can see in every direction.

The horizon isn't a real, physical line you could walk to and touch. It's created by the curve of the Earth and the limits of human vision. If you started walking toward the horizon, it would always stay the same distance away, moving as you move. Sailors navigating across oceans use the horizon to judge distance and direction, and pilots rely on it to keep their aircraft level.

People also use horizon to mean the limit of what someone can experience or understand. A person's horizons are the range of their knowledge and experiences. When you read books about faraway places or learn about new subjects, you're expanding your horizons, which means broadening what you know and can imagine. Someone with broad horizons has experienced many things and remains open to new ideas, while narrow horizons suggest limited experience or unwillingness to consider new possibilities.

When something is “on the horizon,” it means it's coming soon but isn't here yet, like storm clouds gathering on the horizon or exciting opportunities on the horizon in your future.