Glossary of Terms in Journal Citation Reports (extract)

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.
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. 
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 year’s 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.