Open nomenclature involves the use of abbreviated taxonomic expressions in biological classification. The most common expressions used are aff., cf., ?, and sp.. Although there can be variation in where researchers place expressions such as aff. and cf. in the Latin name of a species, the way they are interpreted is the most significant unsettled issue. The International Code of Zoological Nomenclature (ICZN) does not make reference to open nomenclature, leaving its use and meaning open for interpretation by taxonomists.
- The expression aff. is generally used to express affinity of a potentially new but undescribed species with a known species.
- To indicate a potentially new species without showing affinity, sp. is used. This suggests that identification has either not been attempted or the specimen cannot be closely related to established species or subspecies.
- The expressions cf. and ? signify varying degrees or types of uncertainty, with different usage by different authors. "Cf.", for Latin confer, means "compare with", and in more recent usage indicates greater uncertainty (than use of the question mark) in attributing the specimen to the taxon.
- Incertae sedis, a taxon of uncertain position in a classification
- Nomen dubium (nomen ambiguum), a name of unknown or doubtful application
- Species inquirenda, a species that in the opinion of the taxonomist requires further investigation
- Bengtson, Peter (1988). "Open nomenclature" (PDF). Palaeontology 31 (1): 223–227. Archived from the original on 23 January 2011. Retrieved 23 January 2011.
- S. C. Matthews (1973). "Notes on open nomenclature and on synonymy lists". Palaeontoloty 16 (4): 713–719. Retrieved 28 July 2014.