From Halopedia, the Halo wiki
A regiment is a military unit of the UNSC Armed Forces, organized by the Unified Ground Command to structure forces of the UNSC Marine Corps and, rarely, the UNSC Army. In the Marine Corps, these regiments are the primary fighting force seen in sustained operations.
A regiment is composed of three battalions, as well as an addition group of personnel assigned as "Regimental Headquarters Section", totaling to approximately three thousand personnel. While this personnel number is ideal, it is rarely matched exactly in the field. Regiments are traditionally commanded by a colonel, though brigadier generals may hold command over a regiment in some cases.
While a handful of infantry regiments exist in the UNSC Army, the Army makes use of permanent brigades in lieu of regiments, though their unit compositions are largely similar to Marine regiments.
UNSC Marine Corps