Measuring "impact factor" is a way to determine the relative importance of a journal within its field. The higher the impact factor, the more important the journal is compared to others in the same discipline. Impact factors are determined based on calculating the number of citations to articles published in a particular journal. This practice is most commonly used in science/technology and social sciences fields.
More information on Impact Factors see the UW Health Sciences Library guide.
Few tools exist to measure impact factor for Humanities journals. Current tools available--ISI Journal Citation Reports and Eigenfactor--focus primarily on disciplines within science & technology and social sciences. There are two options for cited reference searching in the Humanities: