mitigating circumstance

:  a circumstance in the commission of an act that lessens the degree of criminal culpability was convicted of manslaughter rather than murder because of mitigating circumstances; also :  a circumstance or factor relating to an offense or defendant that does not bear on the question of culpability but that receives consideration by the court especially in lessening the severity of a sentence the mitigating circumstance of the defendant's terminal illness — compare aggravating circumstance