Predatory publishing is an unethical scholarly practice that exploits the "author-pays" publishing model to earn revenue.
The ultimate goal of predatory publishing is monetary gain, not the promotion and preservation of the scholarly record.
They are the publishers that seem to operate as legitimate scholarly journals but use unethical business practices to publish as many papers as possible with little or no consideration of the quality and preservation of the scholarly record.
They charge authors publication fees without providing proper peer review and editorial services offered by legitimate scholarly journals.
NOTE: Not all predatory publishers are deliberately exploitative. Some low-credibility or amateurish publishers that provide no or a superficial amount of peer-review have also been identified as predatory publishers.