In Apple II context, a memory location that contained a pointer to the set of instructions that the machine is to execute when recovering from an INTerrupt generated by the user pressing the RESET button. Typically points by default to the address of DOS on the Apple II+/IIe so as to initiate a disk restart. Sometimes, however, programs leave the reset vector in a strange state before crashing, leaving you with a machine won't do anything except continue to produce errors. Very annoying.