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 like glass. Serenity is from Latin serenus "calm, peaceful" plus the suffix -itas. The first part of the word is the English adjective serene, which means calm and peaceful. The Latin suffix corresponds to our English suffix -ity and has the meaning "quality or state."