In mitigation
Definition 1 of 2
Pro Tip 1/3Legal Context
Most common in law to describe why a punishment should be less harsh. 
The judge considered the defendant's lack of prior offenses in mitigation.
Pro Tip 2/3Not Excusing
Doesn't excuse an action, but offers reasons for leniency. 
He mentioned his troubled childhood in mitigation, not as an excuse for the crime.
Pro Tip 3/3Emotionally Charged
Can carry emotional weight, invoking sympathy or understanding. 
Tears were shed as the family spoke in mitigation at the sentencing hearing.