Plagiarism is the representation of another person's language, thoughts, ideas, or expressions as one's own original work. Although precise definitions vary depending on the institution, in many countries and cultures plagiarism is considered a violation of academic integrity and journalistic ethics, as well as of social norms around learning, teaching, research, fairness, respect, and responsibility. As such, a person or entity that is determined to have committed plagiarism is often subject to various punishments or sanctions, such as suspension, expulsion from school or work, fines, imprisonment, and other penalties.

Not all cultures and countries hold the same beliefs about personal ownership of language or ideas, and plagiarism is typically not in itself a crime. However, like counterfeiting, fraud can be punished in a court for prejudices caused by copyright infringement, violation of moral rights, or torts. In academia and in industry, it is a serious ethical offense. Plagiarism and copyright infringement functionally overlap, depending on the copyright law protection in force, but they are not equivalent concepts, and although many types of plagiarism may not meet the legal requirements in copyright law as adjudicated by courts, they still constitute the passing-off of another's work as one's own, and thus plagiarism.

In the 1st century, the use of the Latin word plagiarius (literally "kidnapper") to denote copying someone else's creative work was pioneered by the Roman poet Martial,^[where?] who complained that another poet had "kidnapped his verses". Plagiary, a derivative of plagiarus, was introduced into English in 1601 by dramatist Ben Jonson during the Jacobean Era to describe someone guilty of literary theft. The derived form plagiarism was introduced into English around 1620. The Latin words plagiārius ("kidnapper") and plagium ("kidnapping") have the same root: plaga ("snare", "net"), which is based on the Indo-European root *-plak, "to weave".

It is frequently claimed that people in antiquity had no concept of plagiarism, or at least did not condemn it, and that 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. Although people in antiquity found detecting plagiarism difficult due to long travel times and scarcity of literate persons, there are a considerable number of pre-Enlightenment authors who accused others of plagiarism and considered it distasteful and scandalous, including historians Polybius and Pliny the Elder. The 3rd century Greek work Lives of the Eminent Philosophers mentions that Heraclides Ponticus was accused of plagiarizing (κλέψαντα αὐτὸν) a treatise on Hesiod and Homer. In Vitruvius's 7th book, he acknowledged his debt to earlier writers and attributed them, and he also included a strong condemnation of plagiarism: "Earlier writers deserve our thanks, those, on the contrary, deserve our reproaches, who steal the writings of such men and publish them as their own. Those, who depend in their writings, not on their own ideas, but who enviously do wrong to the works of others and boast of it, deserve not merely to be blamed, but to be sentenced to actual punishment for their wicked course of life." Vitruvius went on to claim that "such things did not pass without strict chastisement". He recounted a story where the well-read Aristophanes of Byzantium judged a poetry competition and caught most of the contestants plagiarizing others' poems as their own. The king ordered the plagiarizers to confess that they were thieves, and they were condemned to disgrace. Although the story may be apocryphal, it shows that Vitruvius personally considered plagiarism reprehensible.

Although plagiarism in some contexts is considered theft or stealing, the concept does not exist in a legal sense. The use of someone else's work in order to gain academic credit may however meet some legal definitions of fraud. "Plagiarism" specifically is not mentioned in any current statute, either criminal or civil. Some cases may be treated as unfair competition or a violation of the doctrine of moral rights. In short, people are asked to use the guideline, "if you did not write it yourself; you must give credit".

Plagiarism is not the same as copyright infringement. Although both terms may apply to a particular act, they are different concepts, and false claims of authorship generally constitute plagiarism regardless of whether the material is protected by copyright. Copyright infringement is a violation of the rights of a copyright holder, when material whose use is restricted by copyright is used without consent. Plagiarism, in contrast, is concerned with the unearned increment to the plagiarizing author's reputation, or the obtaining of academic credit, that is achieved through false claims of authorship. Thus, plagiarism is considered a moral offense against the plagiarist's audience (for example, a reader, listener, or teacher).

Plagiarism is also considered a moral offense against anyone who has provided the plagiarist with a benefit in exchange for what is specifically supposed to be original content (for example, the plagiarist's publisher, employer, or teacher). In such cases, acts of plagiarism may sometimes also form part of a claim for breach of the plagiarist's contract, or, if done knowingly, for a civil wrong.

There is a journal dedicated to the study of plagiarism, Plagiary: Cross-Disciplinary Studies in Plagiarism, Fabrication, and Falsification.