It is a state of being.

  During the whole of a dull,dark soundless day,in the autumn of that year.When the clouds hung oppressively low in heaven.I had been passing alone on the house's back,through the singularly dreary tract in the country and at length found myself,as the shades of the evening drew on ,within the view of melancholy House of Usher.
  I know not how it was but with the first glimpse of the building,a sense of insufferable gloom pervaded my spirit.I looked upon the scene before me,the simple landscape features of the domain upon the bleak walls,upon the white trunks of decayed trees,with the utter depression souls.
  There was an iciness,a sinking,a sickness of the heart.