This will vary depending on the publisher, so always check before you submit your work.
Here are the guidelines from Echelon Press:
Novel: 50,000- 120,000 words
Short story/novella: 6,000- 50,000 words
Vintage Romance Publishing, LLC considers a novella to be 25,000 words or less. Kensington considers a novella to be 20,000-25,000 words. This range seems to be common among many publishers.
Here are the guidelines from The Scriptorium, a webzine for aspiring writers:
Novel: 45,000-150,000 words
Novella/Novelette: 7,000 - 40,000 words
Short Story: 2,000 - 7,500 words
Short Short: under 2,000 words
Flash Fiction (also known as sudden fiction or micro fiction): under 1000, 500, or even 99 words, depending on the market or guidelines.
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