The concise Oxford dictionary (6th edition) (1976)
Plagiarise - Take and use another person's (thoughts, writings, inventions) as one's own.
The Cambridge international dictionary of English (1995)
Plagiarise - To use (another person's idea or part of their work) and pretend that it is your own.
The Oxford advanced learner's dictionary (5th edition) (1995)
Plagiarise - To take somebody else's ideas or words, and use them as if they were one's own.
The Oxford advanced learner's dictionary (6th edition) (2000)
Plagiarise - To copy another person's words or work and pretend that they are your own.
Collins COBUILD English language dictionary (1987)
If you plagiarise someone else's ideas, or part of a piece of writing or music by someone else, you use it in your own work and pretend that you thought of it or created it.
Funk and Wagnalls' new standard dictionary (1921)
Plagiarism is the act of appropriating the ideas, writings, or inventions of another without due acknowledgement; specifically, the stealing of passages either for word or in substance, from the writings of another and publishing them as one's own.
Collin's pocket English dictionary (1987)
Plagiarism is the taking of ideas, writings, etc. from another and passing them off as one's own
University of Hertfordshire Policies and Regulations, 17.7, 5.2.
Plagiarism is the representation of another person's work as the student's own, either by extensive unacknowledged quotation, paraphrasing or direct copying.
MLA handbook for writers of research papers (1995)
To use another person's ideas or expressions in your writing without acknowledging the source is to plagiarise.