Sector 24 was a region of space, a sector located somewhere in the galaxy's Alpha or Beta Quadrants, and was a location on star charts used by the Federation in the 23rd century.
History and specifics
On reference stardate 1/24, a small probe from the Small Magellanic Cloud was traveling through Sector 24 at warp 30. The USS Greyhound was exploring the sector, which was outside of Federation space at the time. The probe came to a complete stop near the starship. Science officer William Pomeroy studied the now inert object for some time, but was unable to learn anything about it. (ST reference: Spaceflight Chronology)
That probe was Starfleet's first proof of extra-galactic life. (FASA RPG module: The Federation)