terminology - Is it plagiarism to reference a fictitious source?

I'm dealing with a student essay that references three books in support of a claim. None of the books referenced have authors listed, and, based on the contents of the student's essay, I can't find any information on these books online. I'm fairly certain they don't exist, but I'm not sure what to call this beyond academic dishonesty.

I read this from the Nebraska Methodist College:

Other acts of plagiarism are more limited in scope, but are nonetheless cheating. If you decide to make up a quotation or other material and an associated in-text citation, this is plagiarism. If you change or invent the author of a quotation, an idea, or a statistic to make your paper appear to contain more numerous sources, this is plagiarism.

I've run into a few other university plagiarism guides that mention the citing of fictitious sources as plagiarism, but I'd like to know if this is standard, fair, or legitimate to label this practice plagiarism and not just academic dishonesty.

The conflict is this: the student isn't citing these presumably falsified texts directly, but they are referencing finer plot points and characters in them. That is, they are talking about the fictitious dilemmas of the fictitious characters as a way to support their thesis. To me that's dishonest and shows a lack of integrity. I'm grading students on a rubric that awards points for organization, analytical treatment, and language use. If I treat this as academic dishonesty maybe I knock the person's grade down in the rubric criteria related to analysis, but if I treat it as plagiarism I'd give the student a zero.

Answer

For example, Merriam–Webster defines plagiarism via to plagiarize, which it defines as:

: to steal and pass off (the ideas or words of another) as one's own
: use (another's production) without crediting the source intransitive verb

: to commit literary theft : present as new and original an idea or product derived from an existing source

This does not include inventing a quotation, which is in fact sort-of the opposite of plagiarism: passing off one’s own idea as somebody else’s. Other dictionaries agree on this and so does my understanding of the word plagiarism.

Moreover, defining plagiarism so broad makes the term rather useless and almost equivalent to the umbrella term academic misconduct. The reason why we have a word for plagiarism is to differentiate a specific kind of misconduct, not a specific severity.

The conflict is this: the student isn't citing these texts directly, but they are referencing finer plot points and characters. To me that's dishonest and shows a lack of integrity. I'm grading students on a rubric that awards points for organization, analytical treatment, and language use. If I treat this as academic dishonesty maybe I knock the person's grade down in the rubric criteria related to analysis, but if I treat it as plagiarism I'd give the student a zero.

I fail to see why you would be more lenient about academic dishonesty than about plagiarism. I don’t fully understand what you mean by “referencing finer plot points and characters”, but I would classify what you are describing as fabricating evidence, which is roughly as grave as plagiarism. I say roughly, because I see no point in ranking the severity of those misconducts in general and the severity distributions of individual instances of those misconducts strongly overlap.

What is important at the end of the day is whether you are reasonably convinced that the student in question did not just work sloppily, but intentionally deceived the reader (i.e., you). The aspect of intention alone suffices for awarding them zero points, in my opinion.