Plagiarism is a wrongful appropriation of content from another author. Note that it requires two conditions and both of them require judgement.

  1. Appropriation. Copying, or quoting, without citation can be considered appropriation, but not always. It could just be imitation if no implication is made that the copier is attributing the content to themselves. 

  2. Wrongful. The appropriation must be judged to be not allowed. If you start a tale with - *Once upon a time, in a faraway country, there lived a princess* - you are likely to be copying many other authors but it should be okay, since lots of people have used it for far long to be illegal.