The term impact factor is used frequently to denote the stature of an academic journal within a particular discipline. This information can be used as one of the measures when assessing the importance of an individual's published contribution to their research area.
The impact factor is calculated for a journal, based upon the number of citations received by the papers it published over a two year period, divided by the total number of papers published during those years. The calculation is made in the year after publication as that is when the number of citations a paper receives tends to reach a peak.
An example of how this calculation is achieved:
In 2006 there were 51,842 citations to articles published in Nature during 2004 and 2005.
The total number of articles published in Nature during 2004 and 2005 was 1943.
51842 / 1943 = a 2006 impact factor for Nature of 26.681.