Regiment

A regiment is a military unit that in size and organisation is typically between a battalion and a brigade.

Cardassian Union

A khesharo was the Cardassian equivalent to a regiment and was made up of around 5,000 soldiers commanded by a gul. (The Dominion War Sourcebook: The Fires of Armageddon)

Dominion

A regiment in the Dominion is comprised of three battalions numbering 2,187 Jem'Hadar and commanded by a Chelek'atorn. (The Dominion War Sourcebook: The Fires of Armageddon)

Federation Marine Corps

The Federation Marine Corps was comprised of 40 regiments, divided amongst ten divisions, each commanded by a colonel. Each regiment was further subdivided into four battalions of five companies each.(Star Trek: Remington)

Romulan Star Empire

A lagor'val is the Romulan equivalent to a regiment and was formed around a group of tresh'val (battalions) and was between 8,000 - 12,000 soldiers with a colonel in command. (The Dominion War Sourcebook: The Fires of Armageddon)

Starfleet Ground Forces

A Starfleet Ground Forces regiment is between 1,500 and 5,000 Starfleet soldiers (two to five battalions) and is typically commanded by a major or colonel. (LUG RPG: The Dominion War Sourcebook: The Fires of Armageddon)