What is an "impact factor"?
The impact factor of a journal is a measure of the number of citations to recent articles published in that journal. It is used as a reflection 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?
A FREE equivalent measure called the hi-5 index can be found here: http://tinyurl.com/grk4cs2
Impact factors by Journal Citation Reports (paid) can be found here: http://tinyurl.com/p7prujc