And nobody actually gave him a straight answer? Oh wait talon did.
Ignition Control Module, aka ignitor unit. It's the part in the distributor that controls power to the high voltage ignition coil. It takes a low power control signal from the ECU and turns it into an off/on pulse that fires the ignition coil, and in the process puts out a separate pulse signal that the tach uses.
It's about the size of a pack of matches and is screwed to the inside of the distributor. Slo posted a pic of one.