Journal impact factors measure the importance of a particular journal in a field and take into account the number of articles published per year and the number of citations to articles published in that journal. The impact factor is a measure of the frequency with which the average article in a journal has been cited in a particular year. Impact factor is used for journals only.
Impact factor = # of citations to all items published in that journal in the past two years
# of articles and reviews published over those past two years referencing those citations
For example, if a journal has an impact factor of 1.0, this means in the indexed year each article published was cited on average 1 time in the previous two years in that journal.