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 that meet Clarivate Analytics' standards for inclusion in the Web of Science Core Collection are assigned impact factors. Web of Science Master Journal List
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)