A short story is a fictional tale, limited setting, and one major event. It is intended to be read in a single sitting. They are generally under 1000 words. Some people create a number of related short stories and compile them into a novella. There is even a category called micro-fiction, where the word count is under 100 words.
Another popular trend is "flash fiction." These stories are between 250-1000 words. They are particularly popular in the science fiction genre, as they are just short enough to leave the reader with questions to ponder.
All of these are options to consider if you are feeling stuck or have a limited amount of time to write.