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.  For example, if it has been recommended that a Claimant has physiotherapy treatment to aid their recovery and they fail to take out the recommended treatment, then they are at risk of failing to mitigate their losses and may not be fully compensated for the extent of the injuries suffered.