The impact factor of a journal is a quantitative tool for evaluating the relative importance of a journal. It is a measure of the frequency with which its published papers are cited up to two years after publication.
To find out an impact factor use Journal Citation Reports. JCR is available within Web of Knowledge and is the key source of information about the impact of a journal, giving impact factors, cited half-life and immediacy index for each title. JCR covers specialties in the areas of science, technology, and the social sciences and is updated annually in two editions. The Science edition covers over 5,000 journals; the Social Sciences edition covers over 1,500 journals.