Definition: A measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. Clarivate Analytics
Calculation for a journal's 2017 journal impact factor:
Caveat: Only journals in the sciences and social sciences that meet Clarivate Analytics' standards for inclusion in the Web of Science Core Collection are assigned impact factors. Impact factors are not computed for journals in the humanities.
Ongoing Debate: Currently impact factors are the most widely known way to rank journals, however, they are controversial. Some people believe that impact factors do not accurately reflect the impact of a journal or article.
(Hate journal impact factors? New study gives you one more reason, Science July 6, 2016)
Definition: Tthe CiteScore is based on the number of citations to documents (articles, reviews, conference papers, book chapters, and data papers) by a journal over four years, divided by the number of the same document types indexed in Scopus and published in those same four years. Scopus
Available from: Scopus Journal Metrics
Definition: Ranking based on incoming citations for a journal with more weight given to citations from significant and larger journals
Further information: Eigenfactor: Detailed Methods (2007)
Definition: A measure of scientific influence of scholarly journals that accounts for both the number of citations received by a journal and the importance or prestige of the journals where such citations come from. Factors considered for SJR journal rankings include:
Available from: SJR