A short prose literary work with a word count usually set at 2,000-20,000 words, though with considerable variation for upper and lower limits depending on the authority consulted. The Science Fiction and Fantasy Writers of America set the upper word count at 7,499 words, with a novelette being 7,500-17,499 words. Most non-SFWA types would simply call a novelette a long short story. (The distinction is probably just a way for SFWA to give out more story awards.) A prose fanfic story of under 2,000 words would be a ficlet.
In page count, the short story is sometimes pegged to a maximum of 99 pages (per "Novella" on Wikipedia).