An opposing force (alternatively enemy force, abbreviated OPFOR) is a military unit tasked with representing an enemy, usually for training purposes in war game scenarios. The related concept of aggressor squadron is used by some air forces.
At a basic level, a unit might serve as an opposing force for a single scenario, differing from its 'opponents' only in the objectives it is given. However, major armies commonly maintain specialised groups trained to accurately replicate real-life enemies, to provide a more realistic experience for their training opponents.
(To avoid the diplomatic ramifications of naming a real nation as a likely enemy, training scenarios often use fictionalised versions with different names but similar military characteristics to the expected real-world foes.)