The smallest unit of classification can also be stated to be the smallest unit of nomenclature.
A hierarchy of categories, often known as the Linnaean hierarchy, is the classification of organisms into a set order of categories, either in an ascending order (beginning from species and moving up to kingdom) or a descending order (from kingdom to species).\
Kingdom, division or phylum, class, order, family, genus, and species are the seven major obligatory groupings included in the hierarchy.
In this hierarchical classification, species is the smallest unit and kingdom is the largest.