Research impact metrics abound and definitions vary. Below are definitions to some metrics, many derived from Thomson Reuters Journal Citation Reports Glossary.
Altmetrics: Also know as alternative metrics; aims to measure web-driven scholarly interactions on the web, including the number of citations in social media sites as well as open peer or crowd-based recommendations or reviews.
5-Year Journal Impact Factor: the average number of times articles from the journal published in the past five years have been cited in the JCR year. It is calculated by dividing the number of citations in the JCR year by the total number of articles published in the five previous years.
Aggregate Impact Factor: The aggregate Impact Factor for a subject category is calculated the same way as the Impact Factor for a journal, but it takes into account the number of citations to all journals in the category and the number of articles from all journals in the category. An aggregate Impact Factor of 1.0 means that that, on average, the articles in the subject category published one or two years ago have been cited one time.
Article Influence Score: Calculates the relative importance of the journal on a per-article basis and determines the average influence of a journal's articles over the first five years after publication. It is the journal's Eigenfactor Score divided by the number of articles published by the journal. That fraction is normalized so that the sum total of articles from all journals is 1. This measure is roughly analogous to the 5-Year Journal Impact Factor in that it is a ratio of a journal’s citation influence to the size of the journal’s article contribution over a period of five years. The mean Article Influence Score is 1.00. A score greater than 1.00 indicates that each article in the journal has above-average influence. A score less than 1.00 indicates that each article in the journal has below-average influence.
Cited Half-Life: journal cited half-life is the median age of the articles that were cited in the JCR year. Half of a journal's cited articles were published more recently than the cited half-life. Only those journals cited 100 or more times in the JCR year have a cited half-life. A higher or lower cited half-life does not imply any particular value for a journal. The aggregate cited half-life is the median age of the articles that were cited in the JCR year. The aggregate cited half-life is an indication of the turnover rate of the body of work on a subject.
Eigenfactor Score: measures the number of times articles from the journal published in the past five years have been cited in the JCR year. This calculation is similar to the method Google’s PageRank algorithm ranks the influence of webpages; hence, journals are considered influential if they are cited often by other influential journals. Like the Impact Factor, the Eigenfactor is essentially a ratio of number of citations to total number of articles. Unlike the Impact Factor, however, the Eigenfactor counts citations to journals in both the sciences and social sciences and eliminates self-citations.
Google PageRank: evaluates how many links there are to a web page from other pages and the quality of the linking sites.
Google Scholar Metrics: covers a substantial fraction of scholarly articles published in the last 4-5 years; metrics are based on citations from all articles that were indexed in Google Scholar in the current year. See Google Scholar Metrics' overview and coverage information for additional details.
g-index: proposed by Leo Egghe in 2006 to overcome a bias against highly cited papers inherent in the h-index. The g-index is the "highest number of papers of a scientist that received g or more citations, on average" (Schreiber, 2008).
h-index (Hirsch index): measures the impact of a particular scholar rather than a journal. It is defined as the number of papers with citation number higher or equal to h (highest number of publications of a scientist that received h or more citations each while the other publications have not more than h citations each) (Hirsch, 2005). For example, a scholar with an h-index of 5 had published 5 papers, each of which has been cited by others at least 5 times.
Immediacy Index: the average number of times an article is cited in the year it is published. It is calculated by dividing the number of citations to articles published in a given year by the number of articles published in that year. The journal Immediacy Index indicates how quickly articles in a journal are cited. The aggregate Immediacy Index indicates how quickly articles in a subject category are cited.
Journal Impact Factor: measures the importance of a journal and is a measure of the frequency with which the average article in a journal has been cited in a particular year or period. Journal impact factor applies only to a journal or groups of journals, but not to individual articles or individual researchers. The impact factor of a journal in a particular year is the number of citations received in the current year to articles published in the two preceding years divided by the number of articles published in the same two years. For example, Pediatrics has a 2010 impact factor of 5.391, which means that on average each of its 2008 and 2009 articles was cited 5.391 times in 2010. Citing articles may be from the same journal but most citing articles are from different journals.
Median Impact Factor: the median value of all journal Impact Factors in the subject category. The Impact Factor mitigates the importance of absolute citation frequencies. It tends to discount the advantage of large journals over small journals because large journals produce a larger body of citable literature. For the same reason, it tends to discount the advantage of frequently issued journals over less frequently issued ones and of older journals over newer ones. Because the journal impact factor offsets the advantages of size and age, it is a valuable tool for journal evaluation.
Related Journals: the relatedness (R) values derive from a calculation that takes into account the number of citations from the selected journal title, the total number of articles in the related journal, and the total number of citations from the citing journal. Uses the number of citations from one journal to another to determine a relationship.
Self-Citation: Journal Self-Citation is a reference to an article from the same journal. Self-citations can make up a significant portion of the citations a journal gives and receives each year.
SCImago Journal Rank (SJR): a free source that ranks journals and compares journal citation among countries. It expresses the average number of weighted citations received in the selected year by the documents published in the selected journal in the three previous years. This metric doesn't consider all citations of equal weight; the prestige of the citing journal is taken into account.
Source-Normalized Impact per Paper (SNIP): weights citations based on the number of citations in a field. If there are fewer total citations in a research field, then citations are worth more in that field.
Unified Impact Factor: useful when a journal title changes because the impact factor is generally affected for two years. See JCR for how to calculate a Unified Impact Factor.