I'm not sure if the average person is aware, but there are debates within book publishing as petty and pointless as ever the early Catholic Church thought up. In the age of ebooks, some of these debates can become almost of critical importance, because everything one puts up as an ebook is effectively self-described. So how you define things is of great importance, because if you use the wrong term, you may be thought of as wrong (or to pursue the Church analogy, heretical) and punished for your crime (sin).
I speak, of course, of the length of your piece, and how you thus describe it.
There is only two categories, the uninitiated might think: a short story, and a novel. But this would be wrong, and exactly how one defines the stuff in between (or even those two end points) determines the degree of heresy. At the top squats that novel: ponderous, weighty, with no real upper limit, but certainly a lower. What defines a novel is length, few would argue, but what's the bottom edge? Some say 50000 words (which is the definition assigned by NaNoWriMo), and would include just about every book you've ever read. But not quite all; there are a few, like The Old Man and the Sea, that fall below that threshold. And in Young Adult fiction, you can get away with a 40000 word novel and no one blinks. So even that low edge is a bit vague and ragged.
Below the novel but above the short story are several categories poorly defined and uncertain in nature. They are often seen but seldom measured. The novella comes next. Sometimes, it fills the entirety of space between novels and short stories: anything from about 10K to about 40K words. But some people think that it has to be more like 15 or even 20 thousands words to get the novella designation. So under that there is another named category, the novelette. It isn't a popular designation. I don't know anyone who uses it, but the Nebula Awards enjoy it as a category. Maybe they just wanted to be able to give out more awards, like the Best Animated Movie for the Oscars. Anyway, the novelette is sometimes defined as anything above 7500 words, but below 15 or 20 thousand. It's not well known, and not much spotted in nature.
Just for reference, a standard page in a book comes in at maybe 300 words, depending on how much dialogue there is. So a novel must be perhaps 180 or 200 pages; a novella can drop to as little as 30 or so, but the novelette lives in the 25 to 50 pages category overlapping the novella. All right, on to short stories. That should be simple, right?
Nope. There's the short story, which is technically anything that is a story, and that clocks in at under 10K words (or 15, or 20, depending on where your novella category ends). But there are subsets. A short short story is a category that includes anything that's under 1000 words, or about 3 pages at most. But there's the even shorter category of microfiction, which is half that length, or a third: under 500, or maybe 300, words. Flash Fiction is even tinier: perhaps 50 or so words, with one definition stating that it must be 55, no more and no less. And then come the arguments over what is a word: do hyphenates count as one or two (there is disagreement, and it depends on what's hyphenated, is the answer), for instance.
One must be very clear when one defines a work. But one man's clarity is another man's obfuscation. So I think the best choice is to just pick one that feels right, and then state the word count, and then move on with your life. If you've been as clear as you can be, no one can really trash you for misleading them (though I'm told they still, in a few cases, will do so. "This wasn't long enough for the price" is a common complaint, or "I wish I had known how short it was before I bought it".)
To recap, from longest (normally defined) length to shortest: Novel, Novella, Novelette, Short Story, Short Short Story, Microfiction, Flash Fiction. All of them overlapping and blending, causing confusion wherever they are deployed. But that's the way of things, right?