G or g
g is the acceleration of gravity 9.8 (m/s2) or the strength of the gravitational field (N/kg) (which it turns out is equivalent).
When acceleration acts on a physical body, the body experiences the acceleration as a force. The force we are most experienced with is the force of gravity, which causes us to have weight.
The equation for the force of gravity is F = mg, at the surface of the earth, or F = GMm/r2 at a distance r from the center of the earth (where r is greater than the radius of the earth). G is the proportionality constant 6.67x10-11 (N-m2/kg2) in Newton's law of gravity.
When there is an earthquake, the forces caused by the shaking can be measured as a percentage of gravity, or percent g.
For example: The shaking at a particular location is measured as an acceleration of 11 feet per second, or 11*12*2.54 cm/sec/sec = 335 cm/sec/sec. The acceleration due to gravity is 980 cm/sec/sec, so the measured shaking is 335/980, or 0.34 g. As a percentage, this is 34% g.