Journal Impact Factor is from Journal Citation Report (JCR), a product of Thomson ISI (Institute for Scientific Information). JCR provides quantitative tools for evaluating journals. The impact factor is one of these; it is a measure of the frequency with which the "average article" in a journal has been cited in a given period of time.
The impact factor for a journal is calculated based on a three-year period, and can be considered to be the average number of times published papers are cited up to two years after publication. For example, the impact factor 2009 for a journal would be calculated as follows:
A = the number of times articles published in 2008-9 were cited in indexed journals during 2010
B = the number of articles, reviews, proceedings or notes published in 2008-9
impact factor 2010 = A/B