The Impact Factor is a long-standing metric commonly used in research evaluation. It is an equation calculating the average citation frequency for a given journal over a given period of time. It is a ratio of citations to citable items. Generally speaking, the higher the number, the higher the quality and prestige of the journal, although the impact factor is most useful when evaluating journals within the same discipline.
The journal Impact Factor was invented in the 1960s by Eugene Garfield and was intended as a tool to help librarians make selection decisions and authors identify publishing venues. Today, the Impact Factor is a propriety calculation that is available only through Thompson Reuters Journal Citation Reports.
Eigenfactor: A measure of a journal's overall importance to the scientific community based on the origin of incoming citations over a period of time; citations from highly ranked journals are weighed more heavily. (Hosted by the University of Washington; built on Thomson Reuters bibliographic data.)
Journal Metrics: Publicly accessible metrics for journal evaluation that offer three alternative views of true citation impact of a journal. (Provided by Elsevier; built on Scopus bibliographic data.)