Terrain refers to the vertical and horizontal dimensions of land surface. It is usually expressed in terms of the elevation, slope, and orientation of terrain features. The lay of the land affects surface water flow and distribution, and over a large area, it can affect weather and climate patterns. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The understanding of terrain is critical for many reasons, including:

• Military: Precise knowledge of terrain is vital in aviation, especially for low-flying routes and maneuvers, and airport altitudes. Terrain will also affect range and performance of radars and terrestrial radio navigation systems. Furthermore, a hilly or mountainous terrain can strongly impact the implementation of a new aerodrome and the orientation of its runways. The military usage of "terrain" is very broad, encompassing not only landform but land use and land cover, surface transport infrastructure, built structures, and human geography, and even psychological, cultural, or economic factors.

• Weather and Climate: Terrain is important in determining weather patterns. Two areas geographically close to each other may differ radically in precipitation levels or timing because of elevation differences or a rain shadow effect.

Terrain can also refer to a field of knowledge or interest. In times of war, the term "hostile terrain" is used to describe a region with lots of mountains, vast stretches of desert, or impenetrable jungles.