(covers information from several alternate timelines)
This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
The Chief of Starfleet Security was a Starfleet flag officer who oversaw Starfleet Security. In 2367, Vice Admiral Thomas Henry held this post, followed by Admiral Raner in 2370. (TNG: "The Drumhead", "The Pegasus")
In 2371, Benjamin Sisko took the decision to relieve Odo of his duties regarding security for Starfleet operations- Michael Eddington having been appointed head of Starfleet security matters on the station while Odo remained responsible for the station's Bajoran Militia detachment and civilian population- to the Chief of Starfleet Security herself, but was unable to have it overturned. (DS9: "The Search, Part I")
In an alternate 2372, Admiral Strickler was scheduled to have a meeting with the head of Starfleet Security. Strickler noted that she was a person who "doesn't like to be kept waiting." (VOY: "Non Sequitur")