Trinomial nomenclature

In biology, trinomial nomenclature is used to name living things at a rank lower than species. The term "trinomial nomenclature" means "three-part name" or "system of three-part names". 
If living things are divided into more categories than species such as a sub species then it gets a three part scientific name. The first part indicates the genus, the second part indicates the species, and the third part is particular to the living thing (at whatever rank it has been assigned).