Plagiarism -- is the act of using another person's ideas or expressions in your writing without acknowledging the source. It "…is to give the impression that you have written or thought something that you have in fact borrowed from another." (See the MLA Handbook for Writers of Research Papers , [New York: Modern Language Association, 1988], p. 21.)
If you use another person's ideas or expressions without proper citation you have committed plagiarism. It is important that in rewriting you demonstrate your own synthesis of ideas and fully credit your original source. Paraphrasing causes students the most difficulty. When you change words in a sentence, but the idea remains the same, you must cite your source.