Theory of punishment whereby all or part of the purpose of punishment is the infliction of pain or disadvantage on an offender which is in some sense commensurate with his offence and which is inflicted independently of reform or deterrence.
For a weak theory the commensurate amount need not be inflicted but may be, and a limit is placed up to which reformative or deterrent punishment may go but beyond which it may not.
A strong theory insists that the punishment must be inflicted, but again places a limit beyond which it may not go.
Retributivism opposes excessive harshness as much as excessive leniency, and opposes the violation of the offender's rights in the interests of social expediency or personal spite and so on.
Mitigating circumstances, diminished responsibility, and so on are taken into account before determining the commensurate amount, but there are still problems in determining this, and the strong retributivist, especially, must justify violating the presumed moral ban on inflicting unnecessary pain.
J Feinberg, 'The Expressive Function of Punishment', Doing and Deserving (1970)