Plagiarism would be not just copying an idea, plot or even style and structure but basically simply copying the work and attempting to disguise that by changing name, sex, occupations of characters, places or even time.
But even that is acceptable if the source is Public Domain and the author credits the source.
Even when it isn't plagiarism it can be appropriate to credit a source. Some versions of Treasure Island have a credit to R. M. Ballantyne's The Coral Island.
Curiously JK Rowling doesn't credit "The Secret of Platform 13" by Eva Ibbotson, where a boy from a magical Island is treated as a slave by a family and rescued by magical people from the island, who get into London via Platform 13. Nor Jill Murphy's "Worst Witch" series, which even mad it on to TV. But Ibbotson apparently didn't mind.
Ideas, plots and story structure are not copyrightable. Even names can usually be recycled, like a Lawyer called Harry Potter in a book with no magic.
DC Comics have tried to sue people for using the title Supernan, but it's in many books predating the comic.
Wikipedia
Quote:
The word superhero dates back to 1899.[7] Antecedents of the archetype include such mythologic characters like Gilgamesh, Hanuman, Perseus, Odysseus, David, and demigods like Heracles,[8][9] as well as folkloric heroes as Robin Hood, who adventured in distinctive clothing.[10] Real life inspirations behind costumed superheroes can be traced back to the "masked vigilantes" of the American Old West such as the San Diego Vigilantes[11] and the Bald Knobbers[12] who fought and killed outlaws while wearing masks.[13] The French character L'Oiselle, created in 1909, can be classed as a superheroine.[14]
The 1903 British play The Scarlet Pimpernel and its spinoffs popularized the idea of a masked avenger and the superhero trope of a secret identity.
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Of course the US publishers have tried to prevent others using the term.
Plagiarism is deliberate copying of a work and attempting to disguise it as your own. If the work is copyright, then it's a violation of copyright. If it's Public domain it's dishonest unless the original work(s) are credited, such as Clueless by Amy Heckerling loosely based on Jane Austen's 1815 novel Emma, with a modern-day setting of Beverly Hills. (based on Wikipedia).
The Magnificent Seven is based on The Seven Samurai.
Stars originally seems quite based on Dune with a side serving of "The Hidden Fortress", though a court case against Lucas for Plagiarism failed.
Deep Space Nine was written and commissioned after the Studio had studied and rejected Babylon 5, but however much it's inspired by it it doesn't feel like a copy.
Pop music is problematic as there are a limited number of attractive chord progressions and often simple lyrics. Sometimes there is deliberate copying and sometimes it's a co-incidence. Makes lawyers rich.
Book titles are not copyrighted, though a Trade Mark could be registered, though not all are valid if you have deep pockets to fight.