Vexillatio

A vexillatio (: vexillationes) was a detachment of a Roman legion formed as a temporary task force created by the Roman army of the Principate. It was named from the standard carried by legionary detachments, the vexillum (: vexilla), which bore the emblem and name of the parent legion.

Although commonly associated with legions, it is likely that vexillationes included auxiliaries. The term is found in the singular, referring to a single detachment, but is usually used in the plural to refer to an army made up of picked detachments. Vexillationes were assembled ad hoc to meet a crisis on Rome's extensive frontiers, to fight in a civil war, or to undertake an offensive against Rome's neighbours. They varied in size and composition, but usually consisted of about 1000 infantry and/or 500 cavalry.