Pronounced 'gee-sub-cee'.
Basically, this little devil is used to make the units cancel out in the English Engineering system of units. pounds mass can be divided by gc to get slugs.
Defined as gc = (32.2 ft*lbm)/(lbf*s^2).
Keeps engineering students awake at night.
Adding to the confusion, there are two meanings associated with gc--I've heard it applied to both the acceleration due to gravity on the earth (32.2 ft/sec^2, 9.81 m/s^2) and what I have used above, which is to cancel out the units in the English Engineering system of units. It's the ft*lbm/lbf*sec^2 part that's horrible, not the 32.2.