Peace of mind

安心，心定（定心丸）, 放心无忧的状态

例句:

房屋鉴定人彻底察看了房子以后，告诉我们可以放心买了。

The surveyor went over the house thoroughly and advised us that we can buy it with peace of mind.

谁也无法保证事情真相就能带来快乐或是安心。

There is no guarantee that truth will bring happiness or peace of mind.

这不仅没有让她放心，反而带来了更多的麻烦。

Far from giving her peace of mind , it had only created more trouble.

知道所有家人都在夜晚安然入睡真是让人心定。

It is a peace of mind of going to sleep at night with everyone in the family safely in bed.

另外英语口语里面和peace相关的其它短语也有很多，比如：

At peace

不打仗，和平状态

这两个国家处于和平状态。

The two countries were at peace.

In peace

1. 不打扰

你就不能让我静一会吗？

Why can't you leave me in peace?

2. 不想打架

他们说他们来不是想打架的。

They said they came in peace.

Make peace

结束战争

两个国家终于达成和解结束了战争。

The two countries finally made peace (with each other).