Sadness or depression for no evident reason. Usually something insignificant, but can be something big and you just don't realize it.

Adj. Something that is expected to have a long lifespan by virtue of having existed for a long time, based on the lindy effect, named for the New York restaurant Lindy's, coined by Albert Goldman and popularized by Nassim Taleb.

Basically, it's a less intense version of depression, though it differs from it in a lot of ways.

People who are depressed just feel worthless and think that nothing could cure them or make them happy. However, melancholia isn't like that and it's more narcissistic, making the sufferer want 'better' feelings (or better life) - longing for love, the past (if nostalgic) and just want their gaps to be filled. They'd daydream about being with what they long for and such.

Most people with melancholia would think about having a lover and how sad it makes that they still don't have one. They are not depressed (as said) - they cope well with life activities, but they lack that something that fills them up (doesn't have to be a lover though - could be a job, friend, family and even their computer or musical instrument).

NOTE: At times, though, a sustained and intense melancholia may lead to depression.