Background: Plagiarism is one of the most important issues in Research Ethics. The aim of this paper is to discuss about the entity of plagiarism and its kinds. Plagiarism is to appropriate other's work as your own or to use other's works without appropriate citation. The subjects of plagiarism are words, ideas, styles and meanings. This research has tried with descriptive-analytical method to find a comprehensive definition of plagiarism.
Conclusion: Plagiarism may be occurred without intention and the knowledge. The consent of the author of used work doesn't prevent of plagiarism to be occurred. Plagiarism has different kinds that some of them are occurred without any citation to original sources such as stealing the whole or part of a work, stealing the sentences and paragraphs, words and special terms, styles, ideas and the self- plagiarism; and some of them with citation such as Inappropriate paraphrasing, forgotten endnote, writing selected sources and plagiarism from secondary sources.