The MLA Handbook 8th Edition defines plagiarism as the act of "presenting another person's ideas, information, expressions, or entire work as one's own" (6). If you use any part of another person's work in your own writing without properly citing it, this constitutes plagiarism, regardless of if the lack of citation was intentional or accidental.
Plagiarism is a serious academic offense. ATC's Student Handbook outlines punishments for plagiarism up to and including expulsion. Don't let plagiarism happen to you! Familiarize yourself with the common types of plagiarism and some ways to avoid them.