1减少；缩小；降低
They've reduced the prices in the shop, so it's a good time to buy.
商店已经降低了商品的价格，看来现在是买东西的好时候。
2（与to连用）减少至
The fire reduced the forest to a few trees.
大火把森林烧得仅剩下几颗树。
This experienced editor is said to be able to reduce the misprints to almost nil.
据说这位经验丰富的老编辑能把印刷错误几乎降低到零。
3（与to连用）变为，化为
to reduce the rocks to dust
把石块碎成粉末
4（与to连用）强迫；迫使
She was reduced to begging.
她被迫乞讨。
5减肥
1 减轻严重性
Nothing could mitigate the cruelty with which she had treated him.
她对他太残暴了，实在是罪无可恕。
The judge said that nothing could mitigate the cruelty with which the stepmother had treated the girl.
法官说没有任何理由可以把那个继母残忍虐待女孩的罪责减轻。
2 缓和，减低（坏处、伤害等）
3 减轻（痛苦）
to mitigate the sufferings of the dying woman
尽可能减轻这垂死妇人的痛苦
如果非要区别的话，REDUCE是数量上的减少~MITIGATE是程度上的减轻~~~
以上我是摘取自现代汉英词典的翻译~