Adjective
(zoology, botany) Of a subspecies, nominate, having a name which repeats the species name, designating that it represents the originally described population.
Motacilla alba alba is the nominotypical subspecies of the white wagtail (Motacilla alba).
(zoology, botany) Of a subgenus or other infrageneric taxa, having a name which repeats the genus name, indicating that it includes the type species. For older botanical subgeneric and sectional names, it is sometimes prefixed with Eu-.
Source: en.wiktionary.org