Composition
The arrangement of elements within the frame, and the decisions about what to include, exclude and where to place things.
In depth
Composition is fundamentally a process of subtraction: a scene contains everything, a photograph contains only what the edges permit. Every compositional device — thirds, leading lines, framing — is a shorthand for the same underlying question of where the viewer's attention should go and in what order. The camera position and the focal length settle most of it before any rule is consulted.
Examples in practice
Taking three steps left to remove a bin from behind a subject improves a picture more than any rule of thirds adjustment.