There are many journal metrics, measuring citations in a specific journal over time. Here are some common metrics:
Impact Factor: "The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the Journal Citation Reports (JCR) database year" (JCR, 2014).
5-Year Journal Impact Factor: The average number of times articles from the journal published in the most recent 5 years have been cited.
Journal Immediacy Index: The average number of times articles from the journal published in the current year have been cited.
SJR - SCImago Journal Rank: The measure of a journal's scientific influence based on the number of incoming citations and the importance of the journals those citations come from.
SNIP - Source Normalized Impact per Paper: "measures contextual citation impact by weighting citations based on the total number of citations in a subject field." - Scopus
IPP - Impact per Publication: "measures the ratio of ciations in a year to scholarly papers published in the three previous years divided by the number of scholarly papers published in those same years." -Scopus
Eigenfactor: Eigenfactor uses the number of incoming citations to measure a journal’s importance to the scientific community, with citations from higher ranking journals weighted to make a larger contribution than those of poorly ranked journals.
There are many uses for journal impact factors: