Journal Impact Factor
The Journal Citation impact factor is a measure of the relative size of the citation curve in years 2 and 3. It is calculated by dividing the number of current citations a journal receives to articles published in the two previous years by the number of articles published in those same years.
Journal Impact Factor (JIF) is a measure reflecting the average number of citations to articles published in journals, books, patent document, thesis, project reports, newspapers, conference/ seminar proceedings, documents published in internet, notes and any other approved documents. It is measure the relative importance of a journal within its field, with journals of higher journal impact factors deemed to be more important than those with lower ones.

  A = total cites in 2012
  B = 2012 cites to articles published in 2007 - 2011 (this is a subset of A)
  C = number of articles published in 2007 - 2011
  D = B/C = 2012 impact factor

The impact factor is useful in clarifying the significance of absolute citation frequencies. It eliminates some of the bias of such counts which favor large journals over small ones, or frequently issued journals over less frequently issued ones, and of older journals over newer ones. Particularly in the latter case such journals have a larger citable body of literature than smaller or younger journals. All things being equal, the larger the number of previously published articles, the more often a journal will be cited.
An Impact Factor of 1.0 means that, on average, the articles published one or two year ago have been cited one time. An Impact Factor of 2.5 means that, on average, the articles published one or two year ago have been cited two and a half times. The citing works may be articles published in the same journal. However, most citing works are from different journals, proceedings, or books indexed by Web of Science.

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. The median Impact Factor is 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.

Journal Citation Reports (JCR)
Glossary
+ (plus sign)
The plus sign indicates that the citation counts for translated and original language versions of the same journal have been combined.
. (small bullet)
In journal rankings, the small bullet indicates that complete source data were not available for a particular title before the JCR processing deadline (usually mid-February of the JCR year).
Article
An article is a significant item published in a journal covered by Journal Citation Reports®. Editorials, letters, news items, and meeting abstracts are usually not counted as articles because they are not generally cited. The Articles column in the table you see on the Journal Summary page or Journal page counts research articles, review articles, notes, and corrections/retractions. Articles and Reviews are counted separately in the Journal Source Data section of the Journal page.
Citation
A citation is the formal acknowledgment of intellectual debt to previously published research. It generally contains sufficient bibliographic information to uniquely identify the cited document. Information About ...Total Cites, Self-Citations.
Cite
See Citation
Cited Reference
See Citation
Cited Half-Life
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.
Cited Journal Data
Cited Journal data show how many citations a journal received in the JCR year.
Cited-Only Journals (Journals with only Cited Journal information)
Some of the journals listed in JCR are not citing journals, but are cited-only journals. This means that the references in these journals are not included in the database. This is significant when comparing journals because self-citations from cited-only journals are not included in JCR data. Self-citations often represent a significant percentage of the citations that a journal receives. Cited-only journals may be ceased journals, suspended journals, or superseded titles. Any journal that appears elsewhere in JCR, but not in the Citing Journal Listing, is a cited-only journal.
Citing Half-Life
Median age of articles cited by the journal in the JCR year.
Citing Journal Data
Citing journal data show how many citations a journal made to other journals (including itself) in the JCR year.
Immediacy Index
Average number of times an article is cited in the year it is published.
Impact Factor
Average number of times articles from the journal published in the past two years have been cited in the JCR year.
JCR year
The year of the JCR edition displayed in the top right-hand corner of the page. Each JCR year contains one year of citation data. You select the JCR year on the Welcome page.
Journal Title Changes
The Journal Title Changes page lists title changes occurring in the past two years (that is, the JCR year and the preceding year). The listing is in alphabetical order by journal title. To view this list, click the Journal Title Changes link on the Journal page or the Journal Summary List page.
Review
An item is classified as a review if it meets any of the following criteria:

• it cites more than 100 references
• it appears in a review publication or a review section of a journal
• the word review or overview appears in its title
• the abstract states that it is a review or survey

Self-Citation
A 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. You can compare self-citing rates and self-cited rates to supplement your journal evaluation.
Subject Category
Each journal in JCR is assigned to at least one subject category, indicating a general area of science or the social sciences. Journals may be included in more than one subject category; therefore, when comparing journals across related categories, it is possible to see the same journal title in different categories.
Total Cites
The total number of times that a journal has been cited by all journals included in the database in the JCR year.
Citations to journals listed in JCR are compiled annually from the JCR years combined database, regardless of which JCR edition lists the journal and regardless of what kind of article was cited or when the cited article was published. Each unique article-to-article link is counted as a citation.
Citations from a journal to an article previously published in the same journal are compiled in the total cites. However, some journals listed in JCR may be cited-only journals, in which case self-cites are not included.