The Meaning of the "IMPACT FACTOR" of an academic journal...

Journal Impact Factor is a product of Thomson ISI (Institute for Scientific Information) from Journal Citation Report (JCR). 

Journal Citation Report:

JCR provides different quantitative tools to evaluate academic journals and Impact factor is one of those tools. Impact factors are calculated yearly starting from 1975 for those journals that are indexed in the JCR.

Link: http://thomsonreuters.com/journal-citation-reports/

Impact Factor: 

"IMPACT FACTOR is a measure of the frequency with which the "average article" in a journal has been cited in a given period of time."

"In a given year, the impact factor of a journal is the average number of citations received per paper published in that journal during the two preceding years."

Example:

The impact factor 2014 for a journal would be calculated as follows: 

A = the number of times articles published in 2012-2013 were cited in indexed journals during 2013

B = the number of articles, reviews, proceedings or notes published in 2012-2013

impact factor 2014 = A/B

(note that the impact factor 2013 will be published in 2014, because it could not be calculated until all of the 2013 publications had been received. Impact factor 2014 will be published in 2015)

link for top academic journals provided by ISI according to their impact factor.

http://www.cse.chalmers.se/~feldt/advice/isi_listed_se_journals.html