In warfare, a theater, or theatre of war or seat of war is defined as a specific geographical area of conduct of armed conflict, bordered by areas where no combat is taking place.
A theatre is defined by the need for separate planning to be occurring at the highest command echelon of the participating armed forces, including where separate services are concerned. The delineation occurs along regional boundaries or maritime areas that require distinctly separate approach to planning from other regions bordering it. A single conflict may be waged in multiple theaters, and a single nation or an alliance may be participating in multiple theaters. Alternatively a nation may be participating in multiple but unrelated conflicts waged in separate theatres of war.
The a recent example of multi-theatre conflict was the Second World War. This war was waged in two continental war zones, the European Theater and the Pacific Theater, and multiple inter- and intra-connected theaters, often dominated by naval, and sometimes air forces.
Another, more recent, example would be the Portuguese Colonial war which had three separate theaters (Guinea, Angola and Mozambique) and involved over a million men over a period of 10 years.