A cortex bomb is an explosive planted inside a target's skull. It is activated if tampered with, and if it explodes it will naturally kill the bearer. Obviously, this kind of cyberware is highly illegal.
The cortex bomb is a plot element in Shadowrun for the SNES. Jake Armitage learns that, for some reason, he is unable to jack into the Matrix, and visits a street doctor to learn why. Unfortunately for him, this activates the cortex bomb, and Jake has 30 hours to find a doctor capable of disarming and removing it.