"a short novel or long short-story," 1901, from Italian; see novel (n.).

It is not quite so clear as to when and where a piece of fiction ceases to be a novella and becomes a novel. The frontiers are so vague that one is obliged to recognize a middle species, or rather a middle magnitude, which paradoxically, but necessarily enough, we call the novelette. [W.D. Howells, "Some Anomalies of the Short Story," The North American Review, vol. CLXXIII, August, 1901]

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Definitions of novella from WordNet

novella (n.)
a short novel;
Synonyms: novelette