“Plagiarism” describes the copying and imitation of the language, ideas or thoughts of another author. The verbatim and direct copying of texts as well as representations and ideas is a reproduction, and usually an offence. The appropriation of another author’s intellectual performances must be quoted and the source must be acknowledged otherwise it is generally a copyright infringement.
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Self-plagiarism describes copying your own ideas when large parts or structures of previous works, such as a thesis are taken over without marking them as specific quotes. There is debate about whether this also applies when knowledge gained from previous works are treated a second time in another paper. In general the self-appropriation of thoughts challenges the scientific nature of a paper.