closo-

https://doi.org/10.1351/goldbook.C01106
An affix used in names to denote a cage or closed structure, especially a boron skeleton that is a polyhedron having all triangular faces.
Sources:
Blue Book, p. 464 (http://www.acdlabs.com/iupac/nomenclature/)
PAC, 1972, 30, 681. 'Nomenclature of Inorganic Boron Compounds' on page 686 (https://doi.org/10.1351/pac197230030681)
Red Book, 3rd ed., p. 245 (https://doi.org/10.1515/pac-2014-0718)