The Tactical Officer was an officer stationed at the console that controlled a ship's weaponry and shields.
On Starfleet vessels, the tactical officers job was to maintain weapon and shield readiness along with executing firing solutions. These solutions could range from a low intensity phaser beam or to plot a torpedo firing pattern. Security chiefs frequently doubled as tactical officers, though the two roles were not required to be filled by the same person. (TNG: "Pen Pals", "Masks")