定心丸 (dìngxīnwán) something capable of setting somebody's mind at ease; something that smoothes one's nerves; set one's mind at rest

Usage:

“定心丸”是指能让人觉得踏实、稳妥的话语或者行为。

“定心丸”refers to the words or act which can smooth one's nerves.

Sentences:

1、Tīngwán tā de huìbào, wǒ jiù xiàng chī le yì kē dìngxīnwán, zàiyě bù dānxīn le.
听完他的汇报，我就像吃了一颗定心丸，再也不担心了。

I was fully assured and not worry anymore after listening to his report.

2、Xīn de qǐyè wèi zhè ge xiàngmù tóuzī, xiāngdāngyú gěi le xiāofèizhě men yì kē dìngxīnwán.
新的企业为这个项目投资，相当于给了消费者们一颗定心丸。

The new company has invested for the program which has indeed set the customers’ mind at rest.