Plagiarism is "the uncredited use (both intentional and unintentional) of somebody else's words or ideas."
Or more specifically, the "buying, stealing, or borrowing a paper (including copying an entire paper or article from the Web); hiring someone to write your paper for you, and copying large sections of text from a source without quotation marks or proper citation."
Purdue Writing Lab. (n.d.). Is It Plagiarism? // Purdue Writing Lab. Retrieved from https://owl.purdue.edu/owl/research_and_citation/using_research/avoiding_plagiarism/is_it_plagiarism.html.
Plagiarism: the representation of the words or ideas of another as one’s own, including:
1) direct quotation without both attribution and indication that the material is being directly quoted; e.g. quotation marks;
2) paraphrase without attribution;
3) paraphrase with or without attribution where the wording of the original remains substantially intact and is represented as the author’s own;
4) expression in one’s own words, but without attribution, of ideas, arguments, lines of reasoning, facts, processes, or other products of the intellect where such material is learned from the work of another and is not part of the general fund of common academic knowledge;