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H-index was proposed by J.E. Hirsch in 2005 as a numerical characterization of a scholar's output and impact.  Hirsch proposes:  "A scientist has index h if h of his or her N_p papers have at least h citations each and the other (N_p – h) papers have ≤h citations each." 

(N_p)  = number of papers published over n years.

Proc Natl Acad Sci U S A. Nov 15, 2005; 102(46): 16569–16572.
doi:  10.1073/pnas.0507655102)