The Arabic word for tranquility / contentment is طُمَأْنِينَة (tuma'neenah).. This word appears 9 times in the Quran.
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طُمَأْنِينَة
tuma'neenah
tranquility / contentment
بِذِكْرِ اللهِ تَطْمَئِنُّ القُلُوبُ.
Hearts find tranquility in remembrance.