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
Definition: A measure of the average influence of each of a journals' articles over the first five years after publication.