Infrakingdom

Infrakingdom is a taxonomic rank that has appeared in various biological classification schemes. It is used to designate a broad subdivision within a kingdom, intended to group together major lineages that are more inclusive than a subkingdom but more specific than the kingdom as a whole. The exact placement of the infrakingdom, when it is employed, varies by system; in some frameworks it lies between subkingdom and phylum, while in others it is considered a rank above subkingdom or treated as an optional or historical concept.

Historically, infrakingdoms were proposed to reflect large-scale evolutionary relationships within very diverse kingdoms such as animals, plants, or microbes. The concept aimed to provide a formal structure for organizing significant clades that did not fit neatly within a single subkingdom. Over time, and with the adoption of cladistic methods emphasizing monophyly, the use of infrakingdoms has declined in many modern classifications. Today, the rank is encountered less frequently and is not uniformly recognized across major taxonomic references.

In contemporary taxonomy, infrakingdoms are largely considered optional or historical rather than universally accepted, and many systems neither use nor explicitly define this rank. When used, naming conventions for infrakingdoms follow general Latinized practices similar to other high-level ranks, though there is no single, standardized naming scheme.

Etymology and overview: the term combines infra- meaning below with kingdom, signaling a tier below the kingdom level in the traditional hierarchy.