Richard Muller on Permissio and Permissio Efficax

permissio: permission; specifically, permission as distinct from active or effective willing. The concept of a divine permissio was denied by Calvin* but accepted by virtually all later Reformed theologians, including Beza and Zanchi, as a means of explaining the origin of sin and the continuing instances of sin in the course of human history. God does not will positively that sins occur but permits creatures to exercise their will in a sinful way. The concept appears also in the Lutheran scholastic systems, though Lutheran opposition to the doctrine of a decretum absolutum, or absolute decree, of predestination rendered the origin of sin and the continuing presence of sin less of a problem for Lutheran than for Reformed orthodoxy.

permissio efficax: effective permission or willing permission; especially, the providential concursus (q.v.) underlying evil acts of human beings; a concept typical of Reformed theology, which will not allow a bare or ineffectual permission on the part of God and which will acknowledge no realm of activity outside of the will of God. God therefore is viewed as positively willing to permit the free agency of human beings and as supporting their acts with his providential concursus even when those acts go against his revealed will.

Richard Muller, Dictionary of Latin and Greek Theological Terms (Grand Rapids: Baker, 2004), 222.

See also David's posts on Divine Permission of Sin.

*I question whether Muller is correct that Calvin denied the concept of divine permission. Certainly he denied the concept of an unwilling divine permission, but not the concept as such. He probably followed Augustine on the idea. See some Calvin sources here (click).