First off... I will say there is a definite difference between the two but there are people that don't understand the difference between them. And honestly that is what pisses me off when people don't understand it.

Plagiarism in fanfiction is the act of taking something that someone has made and basically taking credit for it. In other words... stealing. You are pretty much taking everything word for word.

Adaptation in fanfiction is taking the original work... using the original plot as a major guideline and putting your own twists into it. And overall somehow making the story your own.

There is a difference between the two. But there are things that people don't understand what could be considered plagiarism and not adaptation.

Paraphrasing - Basically when you paraphrase a story, which means you are taking a story and rewording the entire story. It is considered plagiarism. And please note for those that are still in school.... paraphrasing an entire essay is considered plagiarism as well. It's okay to paraphrase a sentence or two as reference but do not copy the entire thing from your source material.

Adaptation is pretty much using your original source as a structured guideline. In the word of kpop fanfiction... if you are doing an adaptation of something... best way to do it is... basically make it like a fanfiction of your source material. For example... my fic Without U is an adaptation of the manga, Skip Beat. It follows the same plot as Skip Beat verse with my own twists. Now here's the part of adaptation fanfics that you should keep in mind... you are basically writing a FANFIC about your original source material. Adaptation fanfics are pretty much, in a sense, "what if" fanfics. For example... take "West Side Story".. it's an adaptation of Shakespeare's Romeo and Juliet. But if you really think about it... it's the same story of Romeo and Juliet but with it's own twists and more like a "what if" thing throughout the entire story. With adaptations, think of it as a spinoff series of your own. It'll follow the same type of plot or be part of the same verse.... but it'll be also something you created... you are just using that verse as a guideline.

Now do you see why adaptation fanfics are different from plagiarism? You are only supposed to use the original source as a guide. You are NOT supposed to take the entire story and just change the names of the characters to idols or something. I'll give another example, this time of what it would be if something was an adaptation but in reality it's plagiarism....

If you wrote a fanfic using Descendants of the Sun as the original source and stating it's an adaptation but basically you are giving the SAME EXACT plot but changing the names to different people and it's following the plot the same way until the end... THAT IS NOT AN ADAPTATION. That is considered plagiarism. Whenever you make an adaptation MAKE SURE that you make your own twists throughout the story... there are classics such as Pride and Prejudice that actually do have fanfiction adaptations that are published for sale... like the Zombie one... but it has it's own twists.

If I confused you.... feel free to ask any questions and I'll try my best to explain what the difference is between the two.