An Ala (from Latin "ala" meaning "wing") was a mounted auxiliary unit of the Roman army, typically composed of allies (ala sociorum) in later times. The name derives from the traditional use of legionary cavalry on the flanks. The counterparts of alae were first employed during Caesar's Gallic campaigns. Each ala in his army consisted of 10 squadrons. An ala with a strength of 500 horsemen and 504 horses, excluding the prefect, was called "quingenaria," while one with a strength of 1,000 horsemen and 1,090 horses was called "milliaria."
The ala quingenaria was initially divided into 16 squadrons, with each squadron comprising 3 "decuriae" consisting of 10 horsemen and one officer. The ala milliaria consisted of 24 squadrons under the command of a prefect (praefectus alae). According to Gaius Julius Hyginus, a fortified camp could accommodate "4 alae milliariae" and "5 alae quingenariae."
A horseman of the ala was called an "alaris" (plural: alarii). There were also alae serving in the eastern part of the Empire, composed of camel-mounted troops (alae dromedariae).
"Ala, v rímskoi armii" // Brockhaus and Efron Encyclopedic Dictionary: In 86 Volumes (82 Volumes and 4 Supplements). Saint Petersburg, 1890. Volume I, p. 344.