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To plagiarize is to "steal and pass off (the ideas or words of another) as one's own; use (another's production) without crediting the source." (Merriam-Webster's Collegiate Dictionary, 11th ed., 2003)
When you work on a research paper, you will probably find supporting material for your paper from works by others. It's okay to use the ideas of other people and to quote them, but you also need to correctly give credit to them.
It is plagiarism when you:
Buy or use a term paper written by someone else.
Copy and paste passages from the Web, a book, or an article and insert them into your paper without citing them.
Use the exact works or ideas of another person without citing them.
Paraphrase that person's works without citing them.