MLA (Modern Language Association) defines it this way:
"To use another person's ideas or expressions in your writing without acknowledging the source is to plagiarize. Plagiarism, then, constitutes intellectual theft and often carries severe penalties, ranging from failure in a course to expulsion from school."
[MLA Handbook for Writers of Research Papers, 4th ed. p. 26]
APA (American Psychological Association) interprets it:
"An author does not present the work of another as if it were his or her own work. This can extend to ideas as well as written words."
[Publication Manual of the American Psychological Association, 4th ed. p. 294]