Allusion is pointing to a passage in a text by using words from it that are not in quotation marks, but that everyone will recognize, a device used to establish oneself as "in the know" about high literature. But when are you alluding to another text, and when are you in fact plagiarizing from it? Early eighteenth-century writers had an alternative name for "plagiarism": "imitation." Did Alexander Pope plagiarize Virgil's Aeneid in The Dunciad, or was he simply imitating it?
Roger Lonsdale describes a key moment during the eighteenth century in Britain when highly imitative writers began to be accused of plagiarism, that is when notions of copyright ownership had begun to tighten enough to make allusion and imitation over into the crime of plagiarism. Thomas Gray was writing during this moment, and, his borrowings began to be noticed just as the notion of borrowing began to be criminalized, transformed into theft. Gray's poem, "Elegy Written in a Country Churchyard," is so "allusive" that it might be considered the first "hypertext."
Maybe All Great Writers are Copyists
-- A person worked in a small publishing house. S/he selected stories that would be bestsellers for publication, corrected things written by others, and then typed the best passages from manuscripts into a database of great literature. One day, s/he woke up and wrote * / &.
-- "It was coincidental, yet in many ways appropriate, that Imre Kertesz was in Berlin on Oct. 10 when he learned that he had won this year's Nobel literature prize. He had lived off translating German classics during the final 15 years of Communist rule in his native Hungary," NYTimes 12/04/02
Maybe All Great Writers are Plagiarists
-- A person worked for a theater that served to entertain the British monarchy. He was trying to make lots of money, so he found stories already written, copied the characters and events, and imitated or even plagiarized passages from them, altering the style just enough so that the people who came to see his plays would like them better, and so that he would make more money and become more famous.