A taxonomic hierarchy is the sequence of arrangement of taxonomic categories in a descending order during the classification of an organism.

There are seven obligate categories - kingdom, division , class, order, family, genus and species. Species is the lowest category while kingdom is the highest category. The number of common characters is maximum in case of organisms placed in the lowest category.

Number of common characters decreases with the rise in category. Species are the smallest group of individuals which can be recognized by ordinary methods as groups and which are consistently and persistently different from other groups because their characters are less general.