The number of current citations
published in a specific journal
in a two year period divided by the total number of articles published in the same journal in the corresponding two year period.
It should be stressed that a journal's impact factor is a meaningful indicator only when considered in the context of similar journals covering a single field of investigation or subject discipline
. For more information on this, please refer to the essays: The Impact Factor (http://www.isinet.com/isi/hot/essays/journalcitationreports/7.html ) and Using the Impact Factor (http://www.isinet.com/isi/hot/essays/journalcitationreports/8.html)