Melancholia is a state of deep sadness. Your melancholia might make it hard to succeed in your career as a jolly birthday party clown.
Melancholia is a name for a serious, diagnosable mental illness, but it can also mean more of a philosophical or aesthetic idea. You can call severe depression that requires a doctor's care melancholia. But you can also talk about the melancholia of your favorite piece of classical music or novel, if they're imbued with a somber moodiness or a deep sense of sorrow. The Greek root is melankolia, "sadness."