Sadness vs Melancholia - What's the difference?

As nouns the difference between sadness and melancholia

is that sadness is the state or emotion of being sad while melancholia is deep sadness or gloom; melancholy.

(wikipedia sadness)

(uncountable) The state or emotion of being sad.

(countable) An event in one's life that causes sadness.

She has experienced many sadnesses in her forty years.

* (state of being sad) forlornness, melancholy * (event causing sadness) misfortune, woe

Deep sadness or gloom; melancholy

Clinical depression, characterised by irrational fears, guilt and apathy

* melancholiac