Dux
Dux (; plural: duces) is Latin for "leader" (from the noun dux, ducis, "leader, general") and later for duke and its variant forms (doge, duce, etc.).
During the Roman Republic, dux could refer to anyone who commanded troops, including foreign leaders, but was not a formal military rank. In writing his commentaries on the Gallic Wars, Julius Caesar uses the term only for Celtic generals, with one exception for a Roman commander who held no official rank.
Roman Empire
Original usage
Until the 3rd century, dux was not a formal expression of rank within the Roman military or administrative hierarchy.
In the Roman military, a dux would be a general in charge of two or more legions. While the title of dux could refer to a consul or imperator, it usually refers to the Roman governor of the provinces. As the governor, the dux was both the highest civil official as well as the commander-in-chief of the legions garrisoned within the province.
Change in usage
By the mid-3rd century AD, it had acquired a more precise connotation defining the commander of an expeditionary force, usually made up of detachments (i.e. vexillationes) from one or more than one of the regular military formations. Such appointments were made to deal with specific military situations when the threat to be countered with seemed beyond the capabilities of the province-based military command structure that had characterised the Roman Army of the High Empire.
