Mercy is any compassionate act of kindness, lenience, clemency, or aid bestowed upon a distressed animal, person, or persons by someone in power. Charity and humanitarian aid are missions of mercy. The forgiveness of a crime (or reduction of punishment for a crime) could be considered mercy.
The last-resort euthanizing of a sick or injured animal to alleviate its suffering is considered in many cultures to be an act of mercy. Prior to euthanizing, however, all possible comfort, recovery and healing measures must be exhausted.