The impact factor (IF) or journal impact factor (JIF) of an academic journal is a type of journal ranking. Journals with higher impact factor values are considered more prestigious or important within their field.

The impact factor of a journal reflects the yearly mean number of article citations published in the last two years. While frequently used by universities and funding bodies to decide on promotion and research proposals, it has been criticised for distorting good scientific practices.

Impact factor is a scientometric index calculated by Clarivate's Web of Science.

The impact factor was devised by Eugene Garfield, the founder of the Institute for Scientific Information (ISI) in Philadelphia. Impact factors began to be calculated yearly starting from 1975 for journals listed in the Journal Citation Reports (JCR). ISI was acquired by Thomson Scientific & Healthcare in 1992, and became known as Thomson ISI. In 2018, Thomson-Reuters spun off and sold ISI to Onex Corporation and Baring Private Equity Asia. They founded a new corporation, Clarivate, which continued publishing the JCR.

In any given year, the two-year journal impact factor is the ratio between the number of citations received in that year for publications in that journal that were published in the two preceding years and the total number of "citable items" published in that journal during the two preceding years:

$${\displaystyle {\text{IF}}_{y}={\frac {{\text{Citations}}_{y}}{{\text{Publications}}_{y-1}+{\text{Publications}}_{y-2}}}.}$$

For example, Nature had an impact factor of 41.577 in 2017:

$${\displaystyle {\text{IF}}_{2017}={\frac {{\text{Citations}}_{2017}}{{\text{Publications}}_{2016}+{\text{Publications}}_{2015}}}={\frac {74090}{880+902}}=41.577.}$$