An affix used in names to designate three groups occupying the corners of the same face of an octahedral coordination sphere, not now generally recommended for precise nomenclature purposes.
Sources:
Blue Book, p. 464 (https://www.acdlabs.com/iupac/nomenclature/)
Red Book, 3rd ed., p. 245 (https://doi.org/10.1515/pac-2014-0718)