Predatory Publishing: Impact

"deep impact on planet color" by spettacolopuro is licensed under CC BY 2.0

The most well known is the Journal Impact Factor, which is found in the Journal Citation Reports, a database owned by Thompson Reuters  

• Only analyze journals cataloged in ISI Web of Knowledge
• Focus on sciences and social sciences
• Scale varies by field
• Provides a list of the "top journals" in the field

There are other measures of impact with unique calculations

• Citescore (by Scopus)
• CWTS
• h-index or Hirsch index
• Scimago

Impact factors are imperfect measures.  Get the whole picture about a journal.