Gloom is a hazy darkness or dimness. The gloom of a moonless autumn evening is perfect for trick-or-treating, but not so perfect for sizing up your candy stash.
It might make you nervous to walk in the gloom of your friend's rural neighborhood at night if you're used to well-lit city streets. And while gloom perfectly describes the near-dark of dusk or a foggy evening, it can also describe a dark, dismal mood. Your brother's gloom when his favorite basketball team loses in the playoffs will only deepen if your favorite team wins the tournament. Gloom is a Scottish word, originally meaning "a sullen look."