The goal of meditation is to reach a state of serenity, when your mind is still and perfectly calm. Use serenity to mean the quality or state of being calm and peaceful.
It is hard to believe that the ocean, whipped by winds during a storm, can ever return to a state of serenity, where its surface looks as smooth as glass. And if you're babysitting your neighbor's rowdy triplets, you might long for the serenity of your quiet bedroom. Serenity is from Latin serenus, meaning "calm or peaceful," plus the English suffix -ity, meaning "quality or state of."