What are the elements that must be found in any piece of writing for it to be considered a novel?
I think it fair to say that traditional definitions of the word "novel" work along these lines:
A novel is a fictitious prose story of book length.
In my view, a novel has six basic elements:
1) It must have a society of characters;
2) They must be undergoing transformations;
3) And those must be in the course of a sustained prose narrative;
4) The sustained prose narrative must be impelled by a plot;
5) And it must be unified by clearly discernible central themes;
6) And to the extent possible, it must be universalised by a complex architecture of allegorical and symbolical sub-strata.
If I write those six basic elements into a single definition, I come up with this:
A novel is a society of characters, undergoing transformations, in the course of a sustained prose narrative, which is impelled by a plot, unified by clearly discernible central themes, and universalised by a complex architecture of allegorical and symbolical sub-strata.
You won't find that definition in a book, or on anyone else's website (unless they took it from mine, which they're welcome to do).