In military terminology from the twentieth century forward, the corps has been the highest-level military headquarters that has a purely tactical, as opposed to tactical and support/administrative role. In NATO terminology, a corps is an ad hoc unit consisting of two or more divisions (i.e., standing organizations of 10,000-25,000 soldiers each) plus significant corps-controlled attached units). Corps staff historically have been the first level at which planning of operational warfare takes place, although recent improvements in command and control have lowered this level.
Less commonly, corps refers not to a specific tactical organization, but a type of military function such as "medical corps", "corps of engineers", or "tank corps". This meaning is generally synonymous with "branch" or sometimes "department".
This usage may refer to a unit that provides a service as well as the career management path for personnel with that function. In the British Army, the Royal Logistics Corps is both the largest corps organization, but also the staff function that oversees the careers of logistics personnel.
In contrast, the U.S. has three corps concerned with logistics, Quartermaster (i.e., supply), Ordnance (i.e., weapons maintenance and technical support) and Transportation. Transportation Corps personnel are trained through their branch, and transportation doctrine comes from the branch headquarters, but units such as a Transportation Truck Company are part of a combined arms unit.
Some national militaries use "corps" as a level of geographic organization. For example, the Republic of Vietnam (South Vietnam) had four corps, I Corps tactical zone in the northernmost part of the country, III corps near the capital, etc. In July 1970, the Republic of Vietnam redesignated its CTZs as Military Regions (MR). ARVN MR numbers, however, had no relationship to the People's Army of Viet Nam's MR numbering scheme.
To avoid confusion, U.S. corps-sized units operating in Vietnam were called "field forces". Field forces, such as I Field Force Vietnam were tactical headquarters.
In the U.S. military, a corps is also one type of unit of employment (UE), which is an administrative and headquarters organization that controls tactical units of action, which may be Brigade Combat Teams (BCT) or divisions. The term "unit of employment" appears to be going away in favor of the old corps and division titles, but under the new structure introduced with the UE definitions.
Not all militaries, even of large size, used the term "corps". In the Second World War, Japan's equivalent of a corps-sized organization was an "army", and the equivalent to a Western army was an "area army".
The Soviet Union often went from division to army; a Soviet army was often the size of a Western corps. On occasion, the Soviets would create a corps headquarters, usually for a specific purpose.
- eHistory project, Chapter 4: The Summer Campaign in Quang Nam, July-September 1970, Vietnam War archives, University of Ohio