"The action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft."
Source: Oxford English Dictionary
"The unauthorized use or close imitation of the language and thoughts of another author and the representation of them as one's own original work, as by not crediting the author: It is said that he plagiarized Thoreau's plagiarism of a line written by Montaigne. Or something used and represented in this manner: Well-respected publishers are developing a sharper eye for plagiarism in submitted manuscripts."
Source: Dictionary.com
