(1) The illustration, collection, or specimen that represents a taxon. (2) A type specimen. (3) A type material recognized by the International Code of Nomenclature for algae, fungi, and plants (ICN).

For diatoms, many authors designate an individual frustule or valve on a slide as the type specimen. They do this by imaging this specimen and circling it with a diamond objective marker, or marking its coordinates with a device such as an England Finder. While the ICN allows the whole slide containing the type specimen to be designated as the holotype, diatomists tend to avoid this practice in order to minimize later misinterpretation of the intent of the author.

The ICN uses a system called typification that makes objective identification possible by linking the name of a taxon with a holotype. Typification is intended to create a stable system of nomenclature by tying the published name to a holotype or a suite of holotypes.

Several other kinds of type material are recognized by the ICN, including isotype, lectotype, neotype and paratype. Types are not expected to be "typical" or "idealized" specimens, although workers may find such use of the terms.

See also synonym.