As defined by the Brown University Writing Center:
Since it is one of the most dreaded faux pas—in many circumstances it is even considered a crime—in the world of academia and other intellectual circles, plagiarism is a topic that must be addressed by every aspiring writer. Appropriating another person's ideas or words (spoken or written) without attributing those word or ideas to their true source is highly frowned upon in literary and academic circles. Fortunately, with some forethought and common sense, a writer can easily avoid plagiarism. The following resources can help you discern what does—and what does not—constitute plagiarism. Take a look; it is preferable to be safe than sorry when it comes to issues of intellectual property and original thought.
More info: Brown Academic Code & Non-Academic Conduct