to steal and pass off (the ideas or words of another) as one's own: use (another's production) without crediting the source: to commit literary theft: present as new and original an idea or product derived from an existing source. - Merriam-Webster's Collegiate Dictionary, eleventh ed. 2003.
Intentional: deliberate copying or use of another's work without giving credit, submitting a paper from the Internet, another student, or a previous course as one’s own original work, or altering or falsifying citations to hide sources
Unintentional: not properly citing sources, overall sloppy research and note-taking, or cutting and pasting from electronic resources without revision.