In biological taxonomy, circumscription is the definition of a taxon, that is, a group of organisms.
One goal of biological taxonomy is to achieve a stable circumscription for every taxon. Achieving stability is not yet a certainty in most taxa, and many that had been regarded as stable for decades are in upheaval in the light of rapid developments in molecular phylogenetics. In essence, new discoveries may invalidate the application of irrelevant attributes used in established or obsolete circumscriptions, or present new attributes useful in cladistic taxonomy.
An example of a taxonomic group with unstable circumscription is Anacardiaceae, a family of flowering plants. Some experts favor a circumscription in which this family includes the Blepharocaryaceae, Julianaceae, and Podoaceae, which are sometimes considered to be separate families.