What is Mitigation?

Legal Definition
Mitigation in law is the principle that a party who has suffered loss (from a tort or breach of contract) has to take reasonable action to minimize the amount of the loss suffered. As stated by the Canadian Federal Court of Appeal in Redpath Industries Ltd. v. Cisco (The), "It is well established that a party who suffers damages as a result of a breach of contract has a duty to mitigate those damages, that is to say that the wrongdoer cannot be called upon to pay for avoidable losses which would result in an increase in the quantum of damages payable to the injured party." The onus on showing a failure to mitigate damages is on the defendant.

For example, consider a tenant who signs an agreement to rent a house for a year, but moves out (and stops paying rent) after only one month. The landlord may be able to sue the tenant for breach of contract: however the landlord must mitigate damages by making a reasonable attempt to find a replacement tenant for the remainder of the year. The landlord may not simply let the house lie empty for eleven months and then sue the tenant for eleven months' rent.

The actions of the defendant may also result in the mitigation of damages which would otherwise have been due to the successful plaintiff. For example, the Civil Law (Wrongs) Act 2002 (ACT) provides that mitigation of damages for the publication of defamatory matter may result from any apology made by a defendant and any correction published (s. 139I).

The issue of what is reasonable is especially contentious in personal injury cases where the plaintiff refuses medical advice. This can be seen in cases such as Janiak v. Ippolito. The antonym of mitigation is aggravation.
-- Wikipedia
Legal Definition
To make less rigorous or penal.

2. Crimes are frequently committed under circumstances which are not justifiable nor excusable, yet they show that the offender has been greatly tempted; as, for example, when a starving man steals bread to satisfy his hunger, this circumstance is taken into consideration in mitigation of his sentence.

3. In actions for damages, or for torts, matters are frequently proved in mitigation of damages. In an action for criminal conversation with the plaintiff's wife, for example, evidence may be given of the wife's general bad character for want of chastity; or of particular acts of adultery committed by her, before she became acquainted with the defendant; 12 Mod. R. 232; Bull. N. P. 27, 296; Selw. N. P. 25; 1 Johns. Cas, 16: or that the plaintiff has carried on a criminal conversation with other women; Bull. N. P. 27; or that the plaintiff's wife has made the first advances to the defendant, 2 Esp. N. P. C. 562; Selw. N. P. 25. See 3 Am. Jur. 287, 313; Bouv. Inst. Index, h. t.

4. In actions for libel, although the defendant cannot under the general issue prove the crime, which is imputed to the plaintiff, yet he is in many cases allowed to give evidence of the plaintiff's general character in mitigation of damages. 2 Campb. R. 251; 1 M. & S. 284.
-- Bouviers Law Dictionary
Legal Definition
Alleviation; abatement or diminution of a penalty or punishment imposed by law. "Mitigating circumstances" are such as do not constitute a justification or excuse of the offence in question, but which, in fairness and mercy, may be considered as extenuating or reducing the degree of moral culpability. See Heaton v. Wright, 10 How. Prae. (N. Y.) 82; Wandell v. Edwards, 25 Hun (N. Y.) 500; Hess v. New York Press Co., 26 App. Div. 73, 49 N. Y. Supp. 894. See Mitigation of damages.
-- Black's Law Dictionary
Legal Definition
The reduction of damages or punishment by extenuating facts or circumstances.
-- Ballentine's Law Dictionary