Journal’s impact factor
A journal’s impact factor is based on two elements: the numerator, which is the number of cites in the current year to any items published in the journal in the previous 2 years; and the denominator, the number of substantive articles (source items) published in the same 2 years.
Source: Eugene Garfield. The Agony and the Ecstasy—The History and Meaning of the Journal Impact Factor. International Congress on Peer Review And Biomedical Publication Chicago, September 16, 2005