  We study the distributions of citations received by a single publication
within several disciplines, spanning broad areas of science. We show that the
probability that an article is cited $c$ times has large variations between
different disciplines, but all distributions are rescaled on a universal curve
when the relative indicator $c_f=c/c_0$ is considered, where $c_0$ is the
average number of citations per article for the discipline. In addition we show
that the same universal behavior occurs when citation distributions of articles
published in the same field, but in different years, are compared. These
findings provide a strong validation of $c_f$ as an unbiased indicator for
citation performance across disciplines and years. Based on this indicator, we
introduce a generalization of the h-index suitable for comparing scientists
working in different fields.
