Horizon Line
The line where sky meets land or sea, which in a photograph always sits at the camera's own eye level.
In depth
Its height in the frame is a compositional decision with immediate consequences: high gives weight to the foreground, low gives weight to the sky, and dead centre divides the picture into two competing halves. Its levelness is non-negotiable on water, where the eye detects a fraction of a degree of tilt.
Examples in practice
Dropping the horizon to the lower third turns a landscape into a study of a dramatic sky.