Other forms: mitigating circumstances
When a person has done something bad, a mitigating circumstance is a detail that partially justifies it. You ate your sister's cookies, but the fact that you didn't have time for breakfast or lunch is a mitigating circumstance.
Mitigating circumstances don't fully excuse you from a harmful or wrong act, but they help explain it and sometimes mean you'll receive less severe punishment. In legal cases, mitigating circumstances can influence a judge to sentence a convicted criminal to fewer years in prison or a smaller fine. Mitigating is from the Latin mitigare, "soften or soothe," and circumstances has a root meaning "condition."