From an interview in The Paris Review:
What is your definition of a short story?
I think it is the art of the glimpse. If the novel is like an intricate Renaissance painting, the short story is an impressionist painting. It should be an explosion of truth. Its strength lies in what it leaves out just as much as what it puts in, if not more. It is concerned with the total exclusion of meaninglessness. Life, on the other hand, is meaningless most of the time. The novel imitates life, where the short story is bony, and cannot wander. It is essential art.