Clemency is a request for leniency to be granted to a defendant in a criminal trial, wherein the defense attorney requests that the judge consider mitigating factors when sentencing the defendant. Clemency can range anywhere from a reduced sentence to no sentence or house arrest over incarceration. Many people on death row apply for clemency to have the sentence changed to life in prison (this is called commuting the sentence). A judge can issue clemency at any point in a case or during the incarceration period of sentencing. There are some criminals who get clemency after they serve time or even after death, if it is found that the person did not actually commit the crime they were convicted of.
The most common form of clemency is one that is requested of the judge by a criminal prior to sentencing. If a criminal is sentenced to a long period of incarceration or even to death, they can appeal for clemency to the governor of the state they are incarcerated in or even to the president of the country.