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gc (HTML'd g<sub>c</sub>) is the acceleration of gravity at the earth's surface. There's one in the SI (metric system) too:
g = 9.81 m/s^2 = 9.81 N/kg

Yes, gravity does tend to keep Rose-Hulman mechanical engineering students awake at night. Without it, there would be no need for engineers to design airfoils to keep airplanes aloft. What makes it especially horrible in the English Engineering system is that the term pound has been overloaded as both a unit of mass (1/32.2 slug) and a unit of force (1 slug * 1 ft/s^2)