Definition: A measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. Thomson Reuters
Calculation: 2012 impact factor = A/B.
A = the number of times articles published in 2011 and 2012 were cited by journals indexed by Thomson Reuters' Web of Science during 2012.
B = the total number of "citable items" published by that journal in 2011 and 2012
Available from: Journal Citation Reports/JCR
Caveat: Only journals in the sciences and social sciences that meet Thomson Reuters 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 reflex the impact of a journal or article. (Activation Energy Blog, Oct. 22, 2015)
Definition: A measure of the average influence of each of a journals' articles over the first five years after publication.