The 3rd century Greek work Lives of the Eminent Philosophers mentions that Heraclides Ponticus was accused of plagiarizing ( κλέψαντα αὐτὸν) a treatise on Heliod and Homer. Although people in antiquity found detecting plagiarism difficult due to the paucity of literate persons as well as long travel times, there are a considerable number of pre-Enlightenment authors, who accused others of plagiarism and considered it distasteful and scandalous, including the respected historians Polybius and Pliny the Elder. It is frequently claimed that people in antiquity had no concept of plagiarism, or at least did not condemn it, and it only came to be seen as immoral much later, anywhere from the Age of Enlightenment in the 17th century to the Romantic movement in the 18th century. The Latin plagiārius, 'kidnapper', and plagium, 'kidnapping', have the root plaga ('snare', 'net'), based on the Indo-European root *-plak, 'to weave' (seen for instance in Greek plekein, Bulgarian 'плета' pleta, and Latin plectere, all meaning 'to weave'). The derived form plagiarism was introduced into English around 1620. Plagiary, a derivative of plagiarus, was introduced into English in 1601 by dramatist Ben Jonson during the Jacobean Era to describe someone guilty for literary theft. In the 1st century, the use of the Latin word ' plagiarius' (literally 'kidnapper') to denote stealing someone else's creative work was pioneered by the Roman poet Martial, who complained that another poet had 'kidnapped his verses'. Students who move to the United States and other Western countries from countries where plagiarism is not frowned upon may find the transition difficult.

Although some, such as India and Poland, consider plagiarism to be a crime liable for imprisonment, in other countries the reiteration of another professional's work can be a sign of respect or flattery. Not all countries hold the same beliefs about personal ownership of language or ideas. Plagiarism and copyright infringement overlap to a considerable extent, but they are not equivalent concepts, and many types of plagiarism do not constitute copyright infringement, which is defined by copyright law and may be adjudicated on by courts.

In academia and in industry, it is a serious ethical offense. Plagiarism is typically not in itself a crime, but like counterfeiting, fraud can be punished in a court for prejudices caused by copyright infringement, violation of moral rights, or torts. It is subject to sanctions such as penalties, suspension, expulsion from school or work, substantial fines, and even imprisonment. Although precise definitions vary, depending on the institution, such representations are generally considered to violate academic integrity and journalistic ethics as well as social norms of learning, teaching, research, fairness, respect, and responsibility in many cultures. Plagiarism is the fraudulent representation of another person's language, thoughts, ideas, or expressions as one's own original work. Doe, who is writing an essay about the Trojan War, has therefore committed plagiarism by attempting to pass off the writing as his own without sourcing the original article. In this example, the introductory paragraph of the Wikipedia article for the Trojan War (top) has been copy-and-pasted into a Microsoft Word document by John Doe (bottom). A demonstration of how an individual may replicate text from another source to intentionally deceive a reader into believing they wrote the text themselves.

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