Frigate
Frigate is not a term used in the Imperial Navy. Ship of this size and missions within the Imperial Navy are called Escorts.
A Frigate is a class of ships smaller than a Cruiser and with even lighter armament. Frigates are generally smaller than 10,000 tons, with most in the 1,000 to 2,000 ton range. Frigates mount only turret weapons, no spinal or bay mounted weapons.
A Frigate's primary mission is a defensive role. Providing extra security for merchant ships against pirates and light raiders. The frigate generally mounts enough firepower to defeat most armed merchants, but not enough to threaten a Cruiser or similar warship.
The lack of firepower and, generally armor, makes the frigates less expensive to build and maintain than warships, so the can be built and maintained in large numbers.
Their ubiquity lends them to being applied to other missions, including piracy suppression, raiding, and ground support missions.