There is no standard definition of “predatory journal,” a term first used by librarian Jeffrey Beal to describe what he called “counterfeit journals.”
Predatory journals are online free journals that promise to provide quick peer review for submitted articles, but which instead send authors large bills for publishing articles without providing the promised peer review. Predatory journals are claiming that the articles have been reviewed by experts, when they have not.
Often a predatory journal publishes articles with many errors, no review processus, with no article history. They announce fake impact factor and publishing fees don't appear on the instructions.
"Predatory publishers are an even bigger nuisance. Predatory publishers can be defined by a set of characteristics that include lack of peer ‐ review despite claims; dubious quality; or even in some cases charging author fees to make a profit. ... One measure of a journal’s quality has always been its inclusion in a reputable index." Where Do We Go From Here?. Charleston Conference Proceedings.
Please familiarize yourself with the Committee on Publication Ethics (COPE) - a cross-industry initiative that aims to “move the culture of publishing towards one where ethical practices become the norm, part of the publishing culture.” The organization recently consolidated its code of conduct and best practice guidelines for editors and code of conduct for journal publishers into a single, 10-point “core practices” document. It includes cases with advice, guidance for day-to-day practice, education modules and events on topical issues, to support journals and publishers fulfill their policies.