Here's as good a definition as any:
Swinburne University of Technologyís definition of plagiarism
Plagiarism is the action or practice of taking and submitting or presenting the thoughts, writings or other work of someone else as though it is your own work. Plagiarism includes any of the following, without full and appropriate acknowledgment to the original source(s):
(a) the use of the whole or part of a computer program written by another person;
(b) the use, in essays or other assessable work, of the whole or part of a written work from any source including but not limited to a book, journal, newspaper article, set of lecture notes, current or past studentís work, any other personís work, a website or database;
(c) the paraphrasing of anotherís work;
(d) the use of musical composition, audio, visual, graphic and photographic models,
(e) the use of realia, that is objects, artefacts, costumes, models and the like.
Plagiarism also includes the preparation or production and submission or presentation of assignments or other work in conjunction with another person or other people when that work should be your own independent work. This remains plagiarism whether or not it is with the knowledge or consent of the other person or people. It should be noted that Swinburne encourages its students to talk to staff, fellow students and other people who may be able to contribute to a studentís academic work but that where independent assignment is required, submitted or presented work must be the studentís own.
Enabling plagiarism contributes to plagiarism and therefore will be treated as a form of plagiarism by the University. Enabling plagiarism means allowing or otherwise assisting another student to copy or otherwise plagiarise work by, for example, allowing access to a draft or completed assignment or other work.