A resonator coil was a component of a warp core.
In 2368, a Romulan science vessel suffered a forced chamber explosion in their resonator coil as a result of attempting to utilize an experimental interphase generator. This had the effect of completely depolarizing the coil's graviton field generator, rendering it useless. (TNG: "The Next Phase")
After being trapped inside a subspace pocket by a subspace sinkhole in 2375, Tuvok, Tom Paris, and Noss salvaged a pair of resonator coils from one of the ships crashed on their planet. On the way back to their shelter, they were attacked by scavengers and the coils were stolen. (VOY: "Gravity")