η (eta or hapto)

in inorganic nomenclature
https://doi.org/10.1351/goldbook.H01881
An affix giving a topological indication of the bonding between a π-electron ligand and the central atom in a coordination entity. A right superscript numerical index indicates the number of ligating atoms in the π-electron system of the ligand which bind to the central atom. Examples:
  1. [Fe(CO)3(C4H6SO)], tricarbonyl(η2-dihydrothiophene 1-oxide-κO)iron.
  2. [Cr(CO)4(C4H6)], tetracarbonyl(η4-2-methylene-1,3-propanediyl)chromium.
Source:
Red Book, 3rd ed., p. 202 (https://doi.org/10.1515/pac-2014-0718)
See also:
Blue Book, p. 465 (http://www.acdlabs.com/iupac/nomenclature/)
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1120 (https://doi.org/10.1351/pac199466051077)