Journal impact is a measure of the influence that a particular journal has in its field.Researchers often want to publish in journals that have a higher impact factor so that their individual articles have a higher chance of being seen and referenced. The journal’s impact scores do not necessarily indicate anything about the particular authors publishing in it, except that it may be more prestigious to publish in a higher-impact journal.
Many different sources will provide impact metrics for journals and the "best" journal in a field may change depending on the metric. Whenever you see an impact statistic for a journal, you should ask "according to what? How do you know?" Quantitative impact metrics should be used only in conjunction with qualitative measures.
Note: While you often hear the words "journal impact factor" used to indicate the impact of a journal, the actual term Journal Impact Factor (JIF) is a proprietary term that belongs to Clarivate Analytics (formerly Thomson Reuters)/Web of Science. Therefore, if a journal is not indexed in Web of Science, it does not have a Journal Impact Factor, although its impact may be assessed using other metrics.
The tabs on the left will discuss journal impact metrics in various databases. Click the metric names below for more information on how they are calculated. Many impact metrics do not account for differing citation patterns across disciplines, and should not be used to compare journals in different fields. Some impact metrics are normalized to allow comparison across disciplines; those are indicated below by an asterisk after the title.
Also found at eigenfactor.org (Eigenfactor* and Article Influence Score*)
(* metric attempts to normalize in a way that allows comparison across disciplines)