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) [1], "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 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 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[2].