An interdictor is a type of attack aircraft that operates far behind enemy lines, with the express intent of interdicting the enemy logistics effort. The term has generally fallen from use.
The "classic" interdictor is the F-111, an aircraft able to operate at long distances from its base at night, the best time to move up supplies. The F-111 was used during the Vietnam War in the interdictor role, but that was true of almost all United States aircraft during that war.
There were at least two Sith Interdictors defending the Star Forge from Revan and the Old Republic fleet of Admiral Dodonna at the Battle of Rataka Prime (also known as Battle of the Star Forge) during 3,956 B.B.Y. that led to the ending of the Jedi Civil War and the collapse of the Sith Empire.
Interdictor Cruisers built by both the Republic and the Sith possessed 20 quad laser cannons for short range defense, 4 turbolasers, 2 ion cannons, a tractor beam projector, and 4 gravity well generators.
However, the primary purpose of the Interdictor Cruiser was not to be the backbone and powerhouses of both the Old Republic and Sith fleets, but to be a hyperspace interdictor.