There are many ways to describe predatory publishing and there is no standard definition. However, a useful definition of predatory journals and publishers can be found in the December 2019 issue of Nature:
“Predatory journals and publishers are entities that prioritize self-interest at the expense of scholarship and are characterized by false or misleading information, deviation from best editorial and publication practices, a lack of transparency, and/or the use of aggressive and indiscriminate solicitation practice."
It has grown with the development of online publishing and especially open access (OA). In most OA publishing business models, authors are charged fees for publication. This business model is not the problem. Many legitimate and prestigious publishers/journals run the business model successfully. What is problematic is that the predatory journals or publishers exist solely for collecting the funds (Article Publication Charges-APCs) without any commitment to publication ethics or integrity.
Predatory publishers typically contact potential authors directly via email to offer their services and encourage publication, generally in journals or academic conferences.
To the researcher eager to make an impact with their work these can seem like very tempting offers, but they often come with little academic reward.