Euthymia is defined as a normal, tranquil mental state or mood. It is often used to describe a stable mental state or mood in those affected with bipolar disorder that is neither manic nor depressive, yet is distinguishable from healthy controls. Euthymia is also used to describe the “baseline” of other cyclical mood disorders like major depressive disorder (MDD), borderline personality disorder (BPD) and narcissistic personality disorder (NPD). This state is the goal of psychiatric and psychological interventions.