What term is used in biological classification to describe a group of closely related classes?

The term used in biological classification to describe a group of closely related classes is "phylum." In the hierarchical system of taxonomy, the classification begins with broad categories and becomes more specific. The hierarchy can be outlined as follows: domain, kingdom, phylum, class, order, family, genus, and species.

A phylum encompasses multiple classes that share a significant commonality, often reflecting major structural and functional characteristics. For example, the phylum Chordata includes all animals with a notochord, which encompasses several classes such as mammals, birds, and reptiles. This classification emphasizes the evolutionary relationships and shared features among those classes, thereby supporting the concept of biological diversity and evolution.

In contrast, while "order," "family," and "genus" represent lower levels within this hierarchy, they do not capture the same broad grouping of related classes as "phylum" does.