What is Mitigating Circumstances?

Legal Definition
Factors that lessen the severity or culpability of a criminal act, including, but not limited to, defendant's age or extreme mental or emotional disturbance at the time the crime was committed, mental retardation, and lack of a prior criminal record. Recognition of particular mitigating circumstances varies by jurisdiction.
Illustrative caselaw
See, e.g. Magwood v. Patterson, 130 S.Ct. 2788 (2010).
See also
Legal Definition
Activities leading up to an offence that change the perception of why the offence occurred, not necessarily justifying the offence. Family or personal situations are circumstances that may help in obtaining a reduced sentence than is typical for similar offenses. This information may be considered by the courts. It may lead to reducing the accused's degree of culpability or liability. Also refer to aggravating circumstance.
Legal Definition
the term that describes the facts that reduce the security of a crime and its subsequent punishment.