Severe and unrelenting clinical depression; the term depression is generally used over melancholia in diagnosis.
Example: A woman is chronically unable to experience pleasure or to sleep after 4 AM; she finds herself unable to eat and has lost 30 pounds.
Grinnell, R. (2008). Melancholia. Psych Central. Retrieved on April 29, 2016, from http://psychcentral.com/encyclopedia/melancholia/