Created by Professor Henk Moed at CTWS, University of Leiden, Source Normalized Impact per Paper (SNIP) 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.
As explained by Moed in Measuring contextual citation impact of scientific journals, Journal of Informetrics, 4 (2010), pp 256-277:
"It further develops Eugene Garfield's notions of a field's 'citation potential' defined as the average length of references lists in a field and determining the probability of being cited, and the need in fair performance assessments to correct for differences between subject fields."
It is defined as the ratio of a journal's citation count per paper and the citation potential in its subject field. It aims to allow direct comparison of sources in different subject fields. Citation potential is shown to vary not only between journal subject categories – groupings of journals sharing a research field – or disciplines (e.g., journals in Mathematics, Engineering and Social Sciences tend to have lower values than titles in Life Sciences), but also between journals within the same subject category. For instance, basic journals tend to show higher citation potentials than applied or clinical journals, and journals covering emerging topics higher than periodicals in classical subjects or more general journals.
SNIP corrects for such differences. Its strengths and limitations are open to critical debate. All empirical results are derived from the Scopus abstract and indexing database. SNIP values are updated once a year, providing an up-to-date view of the research landscape.
SNIP provides alternative values that bibliometricians can use to create more refined and objective analyses.
It helps editors evaluate their journal and understand how it is performing compared to its competition. SNIP provides more contextual information, and can give a better picture of specific fields, such as Engineering, Computer Science, and/or Social Sciences. It can also help all academics identify which journals are performing best within their subject field so they know where to publish (From: Journal Metrics).