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:
Available from: Journal Citation Reports/JCR
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.
On going 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: CiteScore is the number of citations received by a journal in one year to documents published in the three previous years, divided by the number of documents indexed in Scopus published in those same three years. Scopus
Available from: Scopus Journal Metrics
Definition: A measure of the average influence of each of a journals' articles over the first five years after publication.