According to Journal Citation Reports (JCR), an impact factor is a ratio focusing on original research.
Impact factor = # of citations to all items published in that journal in the past two years
(divided by)
# of articles and reviews published over those past two years referencing those citations
For example, if a journal has an impact factor of 2.5, this means in the indexed year each article published was cited on average 2.5 times in the previous two years in that journal.
Impact factor is used for journals only.
JCR only includes 12,000 journals and conference proceedings from over 3,300 publishers.