Plagiarism is the deliberate attempt to pass off someone else's work as one's own. Therefore, it is the intentional theft of words and/or ideas from a piece of work by someone else. We've said intentional here; it is possible to commit plagiarism accidentally, but this is extremely rare and in the vast majority of cases plagiarists know exactly what they're doing.

Writing a story with a premise similar to that written by someone else -- there, if the earlier story helped to give you the idea, a note to say so would be thoughtful. Using a phrase in a story, even if it appears in someone else's story or novel, where that phrase is common parlance. isn't plagiarism even if it appears in someone else's story. Similarity in plot developments is also very unlikely to be plagiarism. This kind of similarity happens from time to time, and it's often the case that the writer of the later story hasn't even read the earlier one.