In biology a form is a taxonomic subdivision of a species, below the level of a subspecies or variety. The trinomial nomenclature notation is: Genusname speciesname forma formname.

For example a cactus:

• Acanthocalycium spiniflorum forma klimpelianum (Weidlich & Werderm.) Donald

Forms are extensively used to describe subgroupings of plants; they are not used in the taxonomy of animals.

Some botanists believe that there is no need to name forms, since there are theoretically countless numbers of forms based on a single-gene difference.