What is a journal impact factor?
The "impact factor" was created by Dr. Eugene Garfield over 50 years ago as a way to measure the importance or impact of particular journals. It is one of several "quantitative tools for ranking, evaluating, categorizing, and comparing journals. The impact factor ... is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period." (text from The Thomson Reuters Impact Factor essay that is linked below.)
The impact factor is calculated by Thomson Reuters in its Journal Citation Reports each year for the the journals included in its Social Sciences Citation Index and in its Science Citation Index Expanded.
For more information about the origin of the impact factor and how it is calculated, see Essays & White Papers from the Web of Science provided by Thomson Reuters. Particularly,