According to the Oxford English Dictionary, plagiarism is "the action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft." (www.oed.com/view/Entry/144939)
Note that this definition does not include anything about the intent of the author. It's still plagiarism even if it is unintentional.
The Colleges take academic integrity and plagiarism very seriously. These policies are explained in great detail in the: