Trinomial nomenclature is a taxonomic naming system that extends the standard system of binomial nomenclature by adding a third epithet. It is used in biology when the organisms within a species fall into separate groupings that need to be distinguished.

The hierarchy of taxa below the species level is:

• subspecies (ssp./subsp.)
• variety (var./v.)
• subvariety (subvar.)
• form (forma/f.)
• Group or series
• cultivar

Not all are necessarily used for any particular species. In many cases the sub specific groupings are regional. Because of geographical isolation and adaptation to the different circumstances the regional population evolves and over time may become a separate species.

The simplest form of trinomial nomenclature occurs when only a subspecies is being specified. In this case, the subspecies name is added after the species name, without capitalisation but normally typeset in italics. If the genus and species name have already been mentioned in the same paragraph, they are often abbreviated to initial letters: for example one might write, "The Great Cormorant Phalacrocorax carbo has a distinct subspecies in Australasia, the Black Shag P. c. novaehollandiae".

If taxa below the subspecies level are used (which most often arises with plants), an identifying abbreviation is always added. In groups where such lower level divisions are common, it is wiser to indicate when a subspecies name is used, also.

From a scientific point of view, a name is incomplete without an author label and publication details. This specifies who published the name and in what publication and gives the date of the publication in which the name was published.

Cultivated plants can be divided further into groups or series.

When dealing with some genera where there are a lot of cultivars, or where a well-known cultivar becomes variable through poor selection of propagation material or gives rise to new ones through breeding, it has been found useful to use a cultivar-group name to identify them. Such a name always includes the word "Group" and, when used in conjunction with a cultivar name, is enclosed in round brackets. Group names should be capitalized, but not be in italics, or placed between single quotation marks.

Series are named selections of a number of similar cultivars, usually only differing in colour. With seed-raised plants, particularly F1 hybrid flowers, series have become increasingly popular. Series are similar to groups in that they contains a number of similar cultivars, but they differ in being created specifically as a marketing device, with cultivars added to create the wanted range of colours. The identities of individual cultivars are often undisclosed, and the individual colour elements may be replaced by slightly different cultivars over the years. Series names should be capitalized, but not in italics, or placed between single quotation marks. A series name always include the word "Series". If used in conjunction with a cultivar name, the series name should be enclosed in brackets.

Examples

• Phalacrocorax carbo novaehollandiae Stephens, 1826 (no abbreviation is included with animal trinomials, novaehollandis is assumed to be a subspecies, and was described by Stephens in 1826)
• Pinus nigra J. F. Arnold subsp. salzmannii (Dunal) Franco (in plants, it is important to specify that salzmannii is a subspecies, and identifying J. F. Arnold as the author who first described the species European black pine, Dunal as the author who described the species Pinus salzmanii, and Franco as the author who concluded that Pinus salzmanii was only distinct from Pinus nigra at the rank of subspecies)
• Pinus nigra J. F. Arnold var. pallasiana (Lambert) Asch. & Graebn. (if no subspecies name is given, it is assumed to belong to a typical subspecies, in this case Pinus nigra subsp. nigra; and pallasiana is a variety; initially described by Lambert as a species (Pinus pallasiana) and subsequently reduced to a variety of Pinus nigra subsp. nigra).
• Pinus nigra J. F. Arnold subsp. salzmannii var. corsicana (Loudon) Hylander (the Corsican pine is a variety of the western subspecies of the European black pine).
• Adenia aculeata subsp. inermis de Wilde (specifiying that inermis is a subspecies, and identifying de Wilde as the author who named this organism)
• Acanthocalycium klimpelianum var. macranthum (Rausch) J.G.Lamb. (no subspecies name required; macranthum is a variety; initially described by Rausch as a Lobivia and subsequently entered into the genus Acanthocalycium by J.G. Lambert).
• Astrophytum myriostigma subvar. glabrum Backeb. (glabrum is a subvariety name)
• Acanthocalycium spiniflorum forma klimpelianum (Weidlich & Werderm.) Donald (klimpelianum is a form name).
• Alstroemeria L. (Princess™ Series) 'Princess Oxana™' (the Series name in brackets, the cultivar name between quotation marks.)
• Impatiens wallerana Hook f. Tempo Series F1 (Series name without brackets since there are no cultivar name.)

ca:Nomenclatura trinomial