A horizon is a term used in geography to describe the line where the earth's surface and the sky appear to meet. This line can change based on the observer's elevation and the surrounding landscape. For example, standing on a flat plain will provide a different horizon than being on a mountain peak. In photography and art, the horizon line is often used as a compositional element. It helps to create balance and perspective in a scene. The horizon can also be influenced by atmospheric conditions, such as fog or pollution, which can obscure visibility.