There is no one standard definition of what constitutes a "predatory publisher," but generally they are those publishers who charge a fee for the publication of material without providing the publication services an author would expect such as peer review and editing (commercial and non-predatory publishers also often charge authors fees, though they should be more transparent about what purpose those fees serve.) Such a business model aiming for quantity over quality perpetuates bad research in general and exploits both the Open Access publishing model, and academics' need to publish to further their careers.
"Predatory journals: no definition, no defence"
Nature 576, 210-212 (2019)
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