noun

• (psychology, usually uncountable) A state of mind producing serious, long-term lowering of enjoyment of life or inability to visualize a happy future; any of several mental disorders with this state of mind as a central feature.
  "I used to suffer from depression, but now I'm mostly content with my life."
• (psychology, countable) A period of low morale or unhappiness (a period of experiencing the above-mentioned state of mind) which lasts longer than several weeks and may include ideation of self-inflicted injury or suicide.
  "[…] your illness reflects the textbook description of bipolar disorder (euphoric, grandiose, manic highs followed by deep depressions,[…]"
• (geography) An area that is lower in topography than its surroundings.
  "It was not far from the house; but the ground sank into a depression there, and the ridge of it behind shut out everything except just the roof of the tallest hayrick. As one sat on the sward behind the elm, with the back turned on the rick and nothing in front but the tall elms and the oaks in the other hedge, it was quite easy to fancy it the verge of the prairie with the backwoods close by."
• (meteorology) An area of lowered air pressure that generally brings moist weather, sometimes promoting hurricanes and tornadoes.