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)