Singulative

Singulative refers to a noun (or a nominal category of the dimension of number) that denotes a single entity but is expressed by an overt marker and is derived from an unmarked plural-denoting noun (as opposed to singular, which denotes a single entity and is not expressed overtly, or at least not derived from an unmarked plural-denoting noun). The plural-denoting noun from which the singulative is derived is often called collective noun.

Example
Welsh llygod-en 'mouse' is the singulative of the nonderived plural-denoting noun llygod 'mice'.

Origin
The term singulative arose in the linguistics of the Brythonic languages in the 19th century (cf. Cuzzolin 1998) and was later extended to similar phenomena in the Semitic languages.

Reference

 * Cuzzolin, Pierluigi. 1998. Sull'origine del singolativo in Celtico, con particolare riferimento al medio Gallese. Archivio Glottologico Italiano 83.2: 121-149

Other languages
German Singulativ Italian singolativo