Theodicy, a word coined by Leibniz in 1710 from the Greek for god and justice, is the attempt to justify the ways of God in the face of evil. The classical statement of the problem is attributed to Epicurus by Lactantius and was sharpened by Hume in the Dialogues concerning Natural Religion: if God is willing to prevent evil but not able, then God is not omnipotent; if able but not willing, then not good; if both able and willing, then whence evil.
The problem arises with full force only for traditions that affirm a single God who is both all-powerful and wholly good. Dualistic systems distribute the responsibility between two principles; polytheistic systems have gods who are not omnipotent and not uniformly good; and traditions organised around impersonal moral causation face a different question. That the problem is felt so acutely in Judaism, Christianity and Islam is a direct consequence of how much they claim.
Modern discussion works with several distinctions. Moral evil, what agents do, is separated from natural evil, earthquake, disease, predation, because the standard defences apply very unevenly to the two. The logical problem, whether God and evil are strictly incompatible, is separated from the evidential problem, whether the amount and distribution of suffering makes God improbable. Alvin Plantinga's free will defence (1974) is widely held to have answered the logical version, by showing that no formal contradiction follows if free creatures are possible and their misuse of freedom is not something God could prevent while leaving them free. Few philosophers now press the logical problem, and the argument has moved to the evidential version, whose classic modern form is William Rowe's example of a fawn dying slowly in a forest fire that no one witnesses and from which no good is discernibly drawn.
A further distinction is drawn by those who resist the whole enterprise: a defence shows only that God and evil are compatible, while a theodicy claims to say what God's actual reasons are. A number of theologians, and many critics, regard the second as both beyond human competence and morally objectionable, an argument that explaining suffering to those who suffer is not a service to them.