IMPACT FACTOR (IF) - THOMSON REUTERS
"... measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. The annual JCR impact factor is a ratio between citations and recent citable items published. Thus, the impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years"
SOURCE NORMALIZED IMPACT PER PAPER (SNIP) - ELSEVIER
"... measures contextual citation impact by weighting citations based on the total number of citations in a subject field. The impact of a single citation is given higher value in subject areas where citations are less likely, and vice versa."
SCIMAGO JOURNAL RANK (SJR) - ELSEVIER
" is a size-independent indicator and it ranks journals by their 'average prestige per article' and can be used for journal comparisons in science evaluation processes."