These are facts that, while not negating an offence or wrongful action, tend to show that the defendant may have had some grounds for acting the way he/she did. For example, assault , though provoked, is still assault but provocation may constitute mitigating circumstances and allow for a lesser sentence.
MITIGATING CIRCUMSTANCES
Meaning of MITIGATING CIRCUMSTANCES in English
Duhaime's Law English dictionary. Английский юридический словарь Duhaime . 2012