The noun tranquility means "a state of peace and quiet," like the tranquility you feel at the shore of a quiet lake or inside a beautiful cathedral.
Tranquility can also describe a person's disposition. How do you reach tranquility? Make peace with yourself, your life, and the people who drive you crazy. Tranquility can also come from spending time alone, like reading in front of the fireplace on a snowy afternoon. Meditation and yoga can help bring tranquility, by clearing your mind of constant worries.