Serenity literally means the state of being peaceful, calm and clear. Originally this term was used in reference to the weather, signifying a cloudless and bright atmosphere, free of dramatic changes, turbulence and storms. In relation to people, it means the same thing — an absence of perturbation and agitation. Like all the virtues, realizing serenity requires primarily understanding and working with the passion associated with it — in this case, anger.

Sandra Maitri, The Enneagram of Passions and Virtues: Finding the Way Home