What is "wastebasket taxon"

n. (context informal English) A taxon that exists only to classify organisms that do not fit anywhere else, being either paraphyletic or polyphyletic and therefore not considered valid under modern rules of taxonomy.

Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used in some taxonomic circles to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined by either their designated members' often superficial similarity to each other, or their lack of one or more distinct character states or by their not belonging to one or more other taxa. Wastebasket taxa are by definition either paraphyletic or polyphyletic, and are therefore not considered to be valid taxa under strict cladistic rules of taxonomy. The name of a wastebasket taxon may in some cases be retained as the designation of an evolutionary grade, however.