安心 (ān xīn): at ease; to feel relieved; to set one's mind at rest; to keep one's mind on sth

The child was flying alone to another city for the first time; the mother only felt at ease after receiving the news that he had landed.

The doctor said the surgery was successful, so the family members finally felt at ease and went home to rest.

After locking the important documents in the safe, he felt at ease leaving the office to go eat.

You are staying in this quiet room; no one will disturb you at night, so you can prepare for the exam with peace of mind.

When I heard my friend say they had already helped me solve that difficult problem, I immediately felt much more at ease.