An unstable substance which, combined with trilithium and protomatter, formed the explosive carried by a Changeling impostor in an attempt to make the Bajoran sun go nova in By Inferno's Light.
Tekasite is a volatile material named in the Deep Space Nine episode By Inferno's Light in 2373. On its own it is little more than a dangerous cargo, but together with trilithium and protomatter it makes a device capable of triggering a supernova.
The plot is the work of the Dominion. While the real Julian Bashir is held in Internment Camp 371, a Changeling wearing his face steals a runabout from Deep Space 9 and flies it towards Bajor's sun with the three substances aboard. A combined Federation, Klingon and Romulan fleet has gathered at the station expecting a Dominion attack, and the resulting nova would have destroyed the fleet, the station and Bajor together.
The scheme is uncovered at the last moment, and Kira Nerys and Jadzia Dax take the Defiant after the runabout and pull it away with a tractor beam at warp before it reaches the star.