The covert army intelligence unit of the former Soviet Union. GRU is an acronym for Glavnoye Razvedyvatelnoye Upravleniye, roughly the "Central Intelligence Office." This organization existed concurrently with the KGB, with whom it sometimes shared resources, and other times competed. The GRU is headquartered outside Moscow at the Khodinka Airfield, in a 9-story restricted facility referred to as "The Aquarium".
Discussed by Viktor Suvorov, a GRU defector, in his autobiography "Inside the Aquarium."