Everyone agrees that predatory publishers sow confusion, promote shoddy scholarship and waste resources. What is needed is consensus on a definition of predatory journals. This would provide a reference point for research into their prevalence and influence, and would help in crafting coherent interventions.
Grudniewicz, A. (2019). Predatory journals: no definition, no defence. Nature, 576, 210-212, doi: 10.1038/d41586-019-03759-y.

The consensus definition reached by the authors of the paper:
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 practices.
Further details of the main concepts in the definition are included in the article.
Note: Some parts of this article were cross-posted at OpenPhysio, an open-access, peer-reviewed online journal with a focus on physiotherapy education. If you’re doing interesting work in the classroom, even if you have no experience in publishing educational research, we’d like to help you share your stories.