so as to make something, especially a crime, appear less serious and thus be punished more leniently以减轻情况（尤指罪责）的严重程度
柯林斯高阶英汉双解学习词典释义
PHRASE
为减轻罪责；为从轻处置；为了开脱
If someone, especially in a court, is told something in mitigation, they are told something that makes a crime or fault easier to understand and excuse.
例句
Kieran Coonan QC told the judge in mitigation that the offences had been at the lower end of the scale...御用大律师基兰·库南对法官说这些罪行情节较轻，希望能够从宽判决。In mitigation, it should be pointed out that there wasn't much incentive to make world-beating wine.作为开脱的理由也罢，应该指出酿造举世无双的葡萄酒并没有什么实际的好处。
N-UNCOUNT
减轻;缓解;缓和
Mitigation is a reduction in the unpleasantness, seriousness, or painfulness of something.
例句
...the mitigation or cure of a physical or mental condition.身心状况的缓和或痊愈
英英词典释义
Noun
1. to act in such a way as to cause an offense to seem less serious
2. a partial excuse to mitigate censure; an attempt to represent an offense as less serious than it appears by showing mitigating circumstances