A numerus (literally: "number", plural form: numeri) was the term used for a unit of the Roman army. In the Imperial Roman army (30 BC - AD 284), it referred to units of barbarian allies who were not integrated into the regular army structure of legions and auxilia. Such units were of undetermined strength and their organisation and equipment probably varied according to the unit's ethnic origin. The term was also applied to quasi-permanent detachments of regular army units.