The Calendar defines plagiarism as "intellectual theft." The Calendar also says that plagiarism
occurs where an individual submits or presents the oral or written work of another person as his or her own. Scholarship quite properly rests upon examining and referring to the thoughts and writings of others. However, when another person's words (i.e. phrases, sentences, or paragraphs), ideas, or entire works are used, the author must be acknowledged in the text, in footnotes, in endnotes, or in another accepted form of academic citation.
The Oxford English Dictionary gives "PLAGIARY adj. + -ISM suffix." as the term's etymology. That same dictionary says that the adjective is derived from a classical Latin term, "plagiarius", which signifies one who "abducts the child or slave of another," which is itself said to derive from the noun "plagium" which denotes "kidnapping."
An article available from the New York Times Archive entitled "In Defense of Plagiarism" retells an anecdote about "the philosopher Schelling, who complained that Hegel had stolen his ideas: "He was like a shoemaker accusing another shoemaker of having taken his leather and made boots with it. Nothing is more absurd than the assumed right of property in ideas."
The Other One
The Calendar's definition is predicated of the existence of "others" whose literary children one may, as it were, kidnap; of course, when one kidnaps a child, the body of the child is taken away by the kidnapper out of the possession of, as it were, its authors, or parents. Literary theft so-called, however, leaves the possession of the author intact and is therefore a rather extended metaphor.