A novella is a fictional prose narrative, as a rule of thumb more than 20,000 but fewer than 50,000 words in length. These boundaries are approximate, as there is no generally agreed definition of a novella other than that it is of intermediate length. The distinctions between a novella and a short novel, and between a novella and a long short story, are not absolutely clear: but, in general, the novella is of such a length that, published together, no more than four novellas would make a single novel-length book. The vagueness of the description may be seen from the definition offered by Wikipedia, below.

From Wikipedia: A novella is a written, fictional, prose narrative normally longer than a short story but shorter than a novel.

(Note: The situation has been complicated by the use of the term 'novelette' to describe a work intermediate between the long short story and the novella. It seems to have had its origins as an accepted subcategory at the 1955 Hugo Awards, where it designated a work of "between 7,500 and 17,500 words". There is some question as to whether the appearance of the novelette as a separate formal category is simply a commercially-driven attempt to subdivide the traditional short story category for competition purposes: works of this 'novelette' length have been described elsewhere as long short stories. Certainly, the 'novelette' has not been accepted as a distinct form outside genre fiction. Elsewhere, it retains its original, derogatory meaning of a novel of a light, romantic, or sentimental character.)