What does short story mean?

Answer:

A short story means a work of prose fiction that long enough for an average reader to finish it in one sitting rather than several as the longer novel or novella normally takes. This usually means it will be somewhere under 10,000 words.

Edgar Allan Poe once described a short story, or 'tales' as they were then, called as a "...short prose narrative, requiring from a half-hour to one or two hours in its perusal . . . During the hour the hour of perusal the soul of the reader is at the writer's control. There are no external or extrinsic influences resulting from weariness or interruption."

This was taken from a review Poe had made in 1842 of Nathaniel Hawthorne's "Twice Told Tales." This is why it is common to say that a short story should be read in one sitting.