Plagiarism is a wrongful appropriation of content from another author. Note that it requires two conditions and both of them require judgement.
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. Think of well known phrases, a slogan, a well known theorem. In those examples it is clear that if one reproduces them, then one is not implying ownership of the idea. Suppose that you include Veni, vidi, vici! in your text. It is clear for most, who is it that you are citing. It is also clear that you are not claiming being the first using that idea.
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.