Mitigation

n. act of mitigating; appeasement, relaxation; softening in force or severity; lessening the impact or intensity of; condition of becoming less severe; condition of being assuaged (about punishments, circumstances, emotions, etc.)

Noun
1. to act in such a way as to cause an offense to seem less serious
(synonym) extenuation, palliation
(hypernym) decrease, diminution, reduction, step-down
(derivation) extenuate, palliate, mitigate
2. a partial excuse to mitigate censure; an attempt to represent an offense as less serious than it appears by showing mitigating circumstances
(synonym) extenuation
(hypernym) excuse, alibi, exculpation, self-justification
(derivation) extenuate, palliate, mitigate

n. mitigation, appeasement, softening in force or severity

(n.)
The act of mitigating, or the state of being mitigated; abatement or diminution of anything painful, harsh, severe, afflictive, or calamitous; as, the mitigation of pain, grief, rigor, severity, punishment, or penalty.
  

Reasons submitted on behalf of a guilty party in order to excuse or partly excuse the offence committed in an attempt to minimise the sentence