Plagiarism is not a legal concept
Copyright violation is. Patent infringement is. Plagiarism is not.
Plagiarism is academic misconduct - it has no legal definition. It is a matter for the academy in general and any specific institutions involved to deal with.
Just like cheating in a sporting event is not a matter for the law. Lance Armstrong faced no legal sanction for doping, just sanctions within the sport of cycling and the court of public opinion.
What is plagiarism?
Oxford University defines it as:
Plagiarism is presenting work or ideas from another source as your own, with or without consent of the original author, by incorporating it into your work without full acknowledgement. All published and unpublished material, whether in manuscript, printed or electronic form, is covered under this definition, as is the use of material generated wholly or in part through use of artificial intelligence (save when use of Artificial Intelligence - AI for assessment has received prior authorisation e.g. as a reasonable adjustment for a student’s disability). Plagiarism can also include re-using your own work without citation. Under the regulations for examinations, intentional or reckless plagiarism is a disciplinary offence.
Other institutions will have slightly different definitions but the general concept is that plagiarism is failure to give credit where credit is due.
If you credit work and ideas you aren’t committing plagiarism. But you might be committing copyright violation if you copy work without permission even if you credit it.
Similarly, you aren’t committing copyright violation if you have permission, the work is public domain, or are only copying ideas (because ideas don’t have copyright). But you will be committing plagiarism if you don’t credit it.
What is the domain of plagiarism?
Primarily academia. A work submitted for assessment or publication should be free of plagiarism. If it isn’t, that would generally be considered academic misconduct and expose the perpetrator to, at least, criticism and possibly sanction. How strictly plagiarism is enforced depends on both the author and the institution- a professor will be held to a higher standard than an undergraduate who will be held to a higher standard than a high school student and so on. Pre-school students are hardly ever sanctioned for plagiarism.
Secondarily in literature or art. An author or artist whose work is highly derivative (even if not copyright violation) might be accused of plagiarism. For example, at the time of publication, The Lord of the Rings was criticised for plagiarising Wagner’s opera Der Ring des Nibelungen.
Must everything be cited?
Back to Oxford:
If you are substantially indebted to a particular argument in the formulation of your own, you should make this clear both in footnotes and in the body of your text according to the agreed conventions of the discipline, before going on to describe how your own views develop or diverge from this influence.
On the other hand, it is not necessary to give references for facts that are common knowledge in your discipline. If you are unsure as to whether something is considered to be common knowledge or not, it is safer to cite it anyway and seek clarification. You do need to document facts that are not generally known and ideas that are interpretations of facts.
"Money is the third good that everyone wants" definitely needs citation. It’s a pithy little quote and you don’t want people thinking it’s yours. So, despite your best efforts, you can’t find who said it; what do you do?
- Don’t use it.
- Cite it as “Author unknown, circa. 1975”. That way you aren’t claiming credit for it and you give further investigators somewhere to start.
The relative size of heavenly bodies can be considered “facts that are common knowledge in your discipline” as these would be learnt very early in any academic career and can be safely uncited. Of course, any publication that relies on such basic facts is probably not sufficiently “academic” that plagiarism is an issue.