Melancholy

from dictionary.com:

mel·an·chol·y [mel-uhn-kol-ee]

–noun
1. a gloomy state of mind, esp. when habitual or prolonged; depression.
2. sober thoughtfulness; pensiveness.
3. Archaic. a. the condition of having too much black bile, considered in ancient and
medieval medicine to cause gloominess and depression.
b. black bile.

–adjective
4. affected with, characterized by, or showing melancholy; mournful; depressed: a
melancholy mood.
5. causing melancholy or sadness; saddening: a melancholy occasion.
6. soberly thoughtful; pensive.

The word "melancholy" sounds like a beautiful, sad little song in my head. Naturally, I always assumed that's what the word meant: beautiful and sad. I don't really have much to add to this, being in a melancholy mood myself at the moment, but this painting (Proserpine by Dante Gabriel Rossetti) is basically what "melancholy" looks like in my brain. Now you know.