To me, the definition is more structural that length, (I know, I know, organization and critics set their hard and fast rules - but I'm just talking about my own worldview...)
To me, a Novelette has close structural similarities to a short story. A single theme, a closure on that theme. More descriptive background, more narrative complexity that a short story, but a familial resemblance...
A novella, to me, is a novel writen with the terseness of a short story, where ever word counts... Mulitple themes, interweaving connections between them, complex character developement. All in a compact length.
They seem to be different animals, even if their length may overlap. So I consider, for example, Samuel R. Delany's "Time Considered as a Helix of Semi-precious Stones" as a novella, whereas, Fritz Leiber's "Ship of Shadows" (the 1970 Hugo winner for Novella), to be an oversize novelette. <Shrug, you pays your money and makes your choice...>
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