Plagiarism, as defined by the Little, Brown Essential Handbook for Writers, 3rd edition, is "the presentation of someone else's ideas or words as your own. Whether deliberate or accidental, plagiarism is a serious and often punishable offense" (Aaron 133).
Deliberate plagiarism is "copying a sentence from a source and passing it off as your own and, summarizing someone else's ideas without acknowledging your debt, or buying a term paper and handing it in as your own" (Aaron 133).
Accidental plagiarism is "forgetting to place quotation marks around other writer's words, omitting a source citation because you're not aware of the need for it, or carelessly copying a source when you mean to paraphrase" (Aaron 133).
Plagiarize: v. tr. - 1. To use and pass off as one's own (the ideas or writings of another). 2. To appropriate for use as one's own passages or ideas from (another).
v. intr. 1. To put forth as original to oneself the ideas or words of another. (from the American Heritage Dictionary)