What is an "impact factor"?
The impact factor of a journal is a measure reflecting the number of citations to recent articles published in that journal.
It is used as a measure of the relative importance of a journal within its field.
How is it calculated?
X / Y where: X = the number of times that the journal articles were cited in a given year
Y = the total number of "citable items" published by that journal in that same year
Where can I find it?
Impact factors can be found here: http://tinyurl.com/p7prujc