CONCEPT
Plagiarism isn't hard to define. Basically a manuscript should contain only authors' own work, written in their own words. All other instances should comprise appropriate credit to the original authors. Different forms of plagiarism have been categorized(1). This wide-ranged spectrum varies from copying someone else's work until the use of similar wording of previously published material.
PLAGIARISM OF CONTENTS
Using data from one's previous works without proper acknowledgement and credit is perhaps the most harmful form of plagiarism. It includes authorship definition, considering that in an irresponsible list of contributors someone will receive credit for someone else's work(2). Although most of the time misreferencing is a consequence of careless research, it also may be used in order to hide a correct source and therefore appropriate someone's idea.
PLAGIARISM OF FORM
Verbatim plagiarism and paraphrasing are among the most common forms of plagiarism(1). In the first practice, authors copy complete or partial sentences and paste in their own manuscript. Adequate referencing will not prevent verbatim, quotation marks must be included whenever the text being published contains someone else's wording. Paraphrasing demands an extra step of changing the wording after copying someone's idea, without proper citation of the original work. In other words, to avoid plagiarism: if the text contains someone else's wording, it must be written as a direct quote, if it has someone else's ideas, in different wording, it must contain proper credit. Specific algorithms have been perfected to detect verbatim, but they still fail to detect paraphrasing(3).
Although the non-native speaking author burdens a greater effort to write creatively in an original manner, especial attention should be taken to avoid the sequence of copying, pasting and paraphrasing.
SELF-PLAGIARISM
Self-plagiarism accusations often lead to violent reactions from the authors, as they mistakenly assume that they are allowed to reuse their own work as if it was original. Various nuances are involved in this evaluation, including legal consequences of copyright infringement.
When self-plagiarism includes the use of data partially presented elsewhere or the data is "sliced" in different manuscripts, it creates a wrong impression of rich scientific production(4) and may provide wrong information about the prevalence of the findings. This approach has been referred to as "Salami Science"(5).
Editors often struggle with authors that replicate their work, submitting, simultaneously or not, the same manuscript to different journals. The instructions to authors of ABO - Arquivos Brasileiros de Oftalmologia, is very clear on this matter:
"Manuscripts submitted to ABO should not be simultaneously considered for publication by other journals. In addition, total or partial publication or translation for publication in another language of the manuscripts submitted to ABO should not be considered without the permission of the editors of ABO".
RETRACTION VERSUS MISCONDUCT
Scientific community uses retraction as a mechanism to state that a published work should not be considered trustworthy. Retraction occurs if the work contains serious errors, plagiarism or fraud. The former may be related to an error, while the others are considered scientific misconduct.