Novella: a work of fiction intermediate in length and complexity between a short story and a novel.
So here’s the situation. You sat down to write a story. You didn’t think about form or length. You just wanted to tell this particular tale. When you finished, you found that it was 20,000 words long. “Gee,” you thought. “That seems kind of long for a short story.” And then you did a little research, and you found that most literary journals want stories of less than around 5,000 words (some less than 3,000).