It’s a fair question. Just what do we mean when we say a novel is short, or relatively short?
Of course different people would answer differently. But for our purposes, a short novel consists of no more than 80,000 words of the text of the story (exclusive of any critical introductions, notes, and other extraneous matter). It’s hard to say how many pages this ends up being in printed form, because all books are laid out individually, some with many words on each page, some with comparatively few. Usually, however, an 80,000-word novel in printed book form does not exceed 250 pages. So, as a general guideline, it is often the case that novels with printed text of more than 250 pages are in excess of 80,000 words.
We’ve chosen 80,000 words as the cut-off point because there’s something that seems to happen to stories that go on for more than 80,000 words--the length of the book crosses a boundary in terms of scale and scope. For example, Emile Zola’s relatively short novel THERESE RAQUIN (at around 75,000 words, a good test case) seems intimate and condensed when compared to Zola’s much longer masterpieces GERMINAL, NANA, and L’ASSOMMOIR. George Eliot's SILAS MARNER, another short novel pushing the 80,000-word boundary, is a small gem next to her huge novel MIDDLEMARCH. F. Scott Fitzgerald’s THE GREAT GATSBY is compact next to his later book TENDER IS THE NIGHT. Henry James’s THE TURN OF THE SCREW is concise and focused in contrast to THE PORTRAIT OF A LADY.
So 80,000 words is the absolute maximum length of what we accept as a short novel. There is no minimum word count or page count, and many of the books featured on this site are much shorter than 80,000 words. James Cain’s highly respected THE POSTMAN ALWAYS RINGS TWICE has only about 35,000 words, and Marguerite Duras’s autobiographically inspired petite novel THE LOVER has fewer than 30,000 words. And a good number of the books considered classic short novels have fewer than 200 pages (as stated above, page counts can be deceptive, but as a general tendency it’s safe to say that most books under or around 200 pages in length that have relatively easy-to-read font sizes and page layouts would qualify as short novels. Books in the 200-250 page range might take more detective work to estimate the word count, especially if there’s a lot of text per page).
NOTES ON TERMINOLOGY
short novel, novella, and novelette
The words novella and novelette are often used interchangeably with the term short novel, with no appreciable distinction in meaning between the three. This interchangeable meaning is the norm.
In some cases, however, novella and novelette are used to signify subsets within the broader category of the short novel. In this subset usage, novella has the sense that it is a "short short novel" and novelette has the sense that it is a "very short short novel." Thus, if short novel is the genus, novella would be a shorter species (say, 50,000 words or less) and novelette would be an even shorter species (say, 30,000 words or less).
So the three words are used both interchangeably and as subsets within a broader category, depending on the context. But the interchangeable meaning is far more common. You're on shaky ground when you make any kind of clear-cut or rigid distinctions between the three terms, although people sometimes try to do so.
short story versus short novel
Historically, a good number of short stories and short novels appeared in print for the first time in a magazine or some sort of periodical. The practice of serializing novels in magazines or newspapers is much less popular now than it once was, but many great novels, both longer and shorter ones, got their start as serials, divided up over several issues. Short stories still appear frequently in their original print incarnation in literary magazines today, but almost always in one issue.
And when it comes to book publication, especially initial book publication, short stories and short novels often receive sharply different treatment.
A short story usually signifies a story that is not meant to be published by itself in book form. A short story is typically intended to be published as part of something larger--usually as part of a collection of short stories rather than standing on its own in book form.
By contrast, a short novel is usually conceived of as a work that can stand on its own in book form. Almost all the short novels featured on this site were published in book form originally as autonomous volumes. Or, in the rare instances where the short novel was originally published in book form with other fictional works, the author or commentators had some kind of notion of it as a potentially freestanding book, even if it didn't originally turn out that way.
So short novel, novella, and novelette seem to signify a potential for publication in book form as a discrete volume, whereas a short story, being comparatively shorter, seems to almost always be intended as part of a collection of other stories in book form. There are exceptions, as you might guess (for example, the short story "Brokeback Mountain" was republished as a separate volume to tie in with the release of the movie version, and particularly the shorter of the short novels are often included in collections), but in practice the rule usually holds, especially regarding a short novel's initial publication in book form as a stand-alone volume.