Chief tactical officer was a senior position for Starfleet officers on board starships and starbases. Tactical officers were in charge of tactical operations, including weapons and shields, and ship's security.
The Chief tactical officer is also the Chief of security.
On board Constitution-class starships in the early to mid 23rd century, the tactical officer did not have a bridge station, as weapons control was handled by the navigator or helmsman when needed, and security was handled by the ship's security officers. (Star Trek: The Original Series; Star Trek: The Animated Series). However, from the 2270s the tactical station was added to the bridge, from which was controlled the photon torpedoes and deflector screens. (Star Trek: The Motion Picture; Star Trek II: The Wrath of Khan)