An emergency core shutdown is the complete shutdown of the warp core on a Federation starship by means other than regular helm control. This procedure is rarely used, since it leaves the ship in question without warp power for over a week, and as such is designated as an emergency measure only.
Following a collision between the USS Enterprise-D and the USS Bozeman in a temporal causality loop in 2368, an emergency core shutdown was unsuccessfully attempted, and the resultant core breach destroyed the Enterprise, resetting the loop. (TNG: "Cause and Effect")
In 2370, an emergency core shutdown was suggested after the Enterprise began developing severe malfunctions and had gone to warp without command input. However, the vessel dropped out of warp before the procedure could be commenced. (TNG: "Emergence")