Melancholia would still appear to be a far more apt and evocative word for the blacker forms of the disorder, but (Melancholia) was usurped by a noun with a bland tonality... "Brainstorm" has, unfortunately, been preempted to describe, somewhat jocularly, intellectual inspiration. But something along these lines is needed. Told that someone's mood disorder has evolved into a storm--a veritable howling tempest in the brain -- is indeed what a clinical depression resembles like nothing else.
William Styron
There is an element of sadness inherent to both the terms "depression" and "Melancholia". Though the two are even today used as synonyms in some contexts, "Melancholia" has become archaic in this sense of its meaning, and it has become inappropriate to use it as a wholly equal synonym--it is used today as something vaguer and less certain.
The history of the use of the word Melancholia goes back at least as far as 30 A.D., when Aulus Cornelius Celsus first defined it as a "depression caused by black bile". 1,200 years later, Moses Maimonides became the first to declare that Melancholia could be traced to a physical "disease entity"; this is significant, because it marks a surety that this condition is indeed a very human ailment, rather than a natural facet of an individual's personality. That Melancholia, a complex, indecipherable malaise, could become a diagnosis, this implies quite a bit about human nature and science's new-found willingness to explore and define it. Suddenly, humanity was quite certain that there was a "correct" state of mental being, that an individual's "brooding over one particular set of ideas" was something requiring treatment; with this, an individual's unhappiness could now be declared senseless and unjustified, a disorder.
Once an extremely general term denoting depression, "ill-grounded fears, delusions, and brooding over one particular subject", as well as just about any non-immediately-physical ailment, Melancholia today has been differentiated from "psychotic depression" by the fact that psychotic depression tends to occur in people who are "worriers" and whose "stability under stress (is) tenuous". Also, psychotic depression, unlike Melancholia, is often marked by psychomotor disturbance, constipation, and "morbid cognitions (involving guilt and a sense of deserving punishment)".
Melancholia is a form of depression in which either no symptoms of mania exist, or no symptoms of mania have yet been recognized. The operative word in the understanding of the term is provided by Webster: "brooding", "to focus the attention on a subject persistently and moodily."
http://www.bipolarhome.org/understanding.html
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9105957&dopt=Abstract
http://www.dictionary.com/cgi-bin/dict.pl?term=brooding | 
