A category whose members are organised with respect to a composite prototype. The members of the radial category are not generated. Rather, they are extended by convention and therefore must be learned. The composite prototype determines the possibilities for the extensions, together with the possible relations between variants and the central prototype.

An important line of research in cognitive lexical semantics has been to treat words as radial categories organised with respect to a composite prototype. The most famous work in this vein has been carried out on the English preposition over. Radial categories are modelled in terms of a semantic network.