Calm is the absence of stress. Whenever life becomes a taskmaster and chases us with its ever-increasing demands, what we experience is an inner agitation. And calm is that inner clarity we feel when we are not stuck in a whirlwind of emotions.
Calm, is the sereneness you can find with a small practice of meditation or even listening to soothing music. It is a state where your limbic system is not triggered unnecessarily. But every one of us who has little observation would have been able to see by now, that this state of calm can be easily toppled by stress. Calm often lacks a firm founding in wisdom. Like the full moon playing hide-and-seek from behind the clouds, your mind easily vacillates between periods of calm and stress depending on how life evolves externally.