A species that has been selected as the standard bearer of a genus or subgenus
(ICZN) the species that is the name-bearing type of a genus or subgenus. (In contrast, according to the ICBN the type of a genus or subgenus is a specimen.)
The nominal species that is the name-bearing type of a nominal genus or subgenus.
A type species is the species of an organism from which a new genus is named. For example, Tyrannosaurus rex is the type species for the genus Tyrannosaurus.
A type species fixes the name of a genus (or of a taxon in a rank lower than genus).