A regiment was a military unit. Depending on the, branch of service, mission, method of organization, and the sanctioning or parent government, a regiment typically consisted of a variable number of subordinate units and was commanded by a field grade officer.
In the mid-22nd century, Commander Dolim commanded a regiment of the Xindi-Reptilians. The Reptilian, who piloted the Xindi superweapon prototype in 2153 was from Dolim's regiment, personally selected by him. (ENT: "Azati Prime")
When General Martok seemed to recognize Miles O'Brien, disguised as the Klingon Pahash in 2373, O'Brien claimed that his regiment had fought in the Battle of Mempa, where the two must have met. (DS9: "Apocalypse Rising")