melancholy vs sadness

melancholy

noun
  • Great sadness or depression, especially of a thoughtful or introspective nature. 

  • Black bile, formerly thought to be one of the four "cardinal humours" of animal bodies. 

adj
  • Affected with great sadness or depression. 

sadness

noun
  • The state or emotion of being sad. 

  • An event in one's life that causes sadness. 

How often have the words melancholy and sadness occurred in a corpus of books? (source: Google Ngram Viewer )