The Kalandra sector is a region of Federation space near the core Federation worlds. The Kalandra system is located within it.
In 2374, Dominion forces launched a successful invasion of Betazed from the Kalandra sector, which Starfleet Intelligence had believed too far from their supply lines to be a threat. (DS9: "In the Pale Moonlight") The sector subsequently fell to the Dominion, allowing them to threaten Vulcan. (DS9: "The Reckoning")
Based on Cardassian military transmissions decoded by Elim Garak, Starfleet launched a counter-offensive and retook the Kalandra sector in early 2375. (DS9: "Afterimage")
The sector later served as the rendezvous point after General Martok's raid on Trelka V, where the USS Defiant and seven other Federation starships stood in wait for the returning Klingon forces. (DS9: "Once More Unto the Breach")