Series is a low-level taxonomic rank below that of section (and subsection) but above that of species.
In botany and plant taxonomy, a series is a subdivision of a genus. It is typically used to help organize very large genera, which may have hundreds of species.
A series gets a two part name, or binary name, consisting of the name of the genus and an epithet.
The term "series" is also used (in seed marketing) for groupings of cultivars, but this term has no formal status in the ICNCP.