Wikipedia
Zersetzung (German; variously translated as decomposition, corrosion, undermining, biodegradation or dissolution) is a psychological technique originally devised and used by the East German secret police, the Stasi, to silence political opponents. The "measures of Zersetzung", defined in the framework of a directive on police procedures in 1976, were used in the context of so-called "operational procedures" (in German Operative Vorgänge or OV). They replaced the overt terror of the Ulbricht era.
The practice of repression in Zersetzung comprised extensive and secret methods of control and psychological manipulation, including personal relationships of the target, for which the Stasi relied on its network of informal collaborators, (in German inoffizielle Mitarbeiter or IM), the State's power over institutions, and on operational psychology. Using targeted psychological attacks the Stasi tried to deprive a dissident of any chance of a "hostile action".
The use of Zersetzung is well documented due to numerous Stasi files published after East Germany's Wende. Several thousands or up to 10,000 individuals are estimated to have become victims 5,000 of whom sustained irreversible damage. Pensions for restitution have been created for the victims.