Academic honesty or academic integrity is a set of values held by the academic community. These values are defined by the International Center for Academic Integrity as "commitment, even in the face of adversity, to...honesty, trust, fairness, respect, and responsibility."
There are four basic types of academic dishonesty: cheating, facilitation, plagiarism, and fabrication. Sometimes plagiarism is used as a catch-all term for all kinds of academic dishonesty. Plagiarism is using somebody else's ideas or expression of ideas (writing, art, music, speech, etc.) and representing them as your own.
More specifically, plagiarism is the "unattributed use of a source of information that is not considered common knowledge. Three acts are considered plagiarism: (1) failing to cite quotations and borrowed ideas, (2) failing to enclose borrowed language in quotation marks, and (3) failing to put summaries and paraphrases in your own words."
~~Diana Hacker, A Pocket Style Manual
adapted from UW Bothell's Academic Integrity and Plagiarism Prevention Resources