A subnucleonic device was a device of unknown design that could be used to damage ore processors.
In 2365, a subnucleonic device was used to damage an ore processor on Level 21 of Terok Nor, taking it out of operation for two weeks. Kira Nerys of the Bajoran Underground claimed responsibility to causing the sabotage, but admitted to Odo in 2370 that her colleague actually damaged the processor, as her responsibility was to find Vaatrik's list of Bajoran conspirators – an action that ultimately resulted in Kira murdering Vaatrik. (DS9: "Necessary Evil")
It is unclear what a subnucleonic device looks like or how it functions. Kira claimed she could describe the device to Odo, but was never asked to.