noun: a feeling of thoughtful sadness
noun: a constitutional tendency to be gloomy and depressed
noun: a humor that was once believed to be secreted by the kidneys or spleen and to cause sadness and melancholy
adjective: characterized by or causing or expressing sadness ("Growing more melancholy every hour")

Hughes
If we strike any major threefold chord, and directly afterwards its relative minor, we may notice how they respond to the twofold natures within us of joy and melancholy. [Harmonies of Tones and Colours, Reflections on the Scheme2, page 44]

"Joy and melancholy, virtue and vice, are as much the consequences of natural law as the falling of a stone or the growth of a flower."—C. Watts. [Harmonies of Tones and Colours, Reflections on the Scheme2, page 44]

"All things are touched with melancholy,
Born of the secret soul's mistrust
To feel her fair ethereal wings
Weighed down with vile degraded dust.
There is no music in the life
That sounds with idiot laughter solely;
There's not a string attuned to mirth,
But has its chord in melancholy."
Thomas Hood. [Harmonies of Tones and Colours, Reflections on the Scheme3, page 45]