H-index

The h-index measures a researcher's combined productivity and citation impact as a single number. It is defined as the largest number h such that the researcher has h publications that have each been cited at least h times; a scholar with an h-index of 20 has 20 papers cited at least 20 times each, while their remaining papers have fewer citations. It was proposed by physicist Jorge Hirsch in 2005.

This differs from simply counting total citations or total papers, because the h-index rewards a body of work that is both productive and consistently cited: one blockbuster paper with thousands of citations cannot by itself push the index very high if the rest of the output is thin. It also differs from the journal impact factor, which rates a journal's average citation rate rather than an individual's.

The h-index is one input among many in bibliometrics used for hiring, tenure and grant decisions, but it has well-documented limits: it depends heavily on field citation norms and career length, so it cannot fairly compare researchers across disciplines or career stages, it does not account for self-citation or author order, and it only increases or holds steady over a career, never falling, even as older work becomes outdated.