Denotes gravity in mathematical notation.
Specifically, g is the acceleration due to the local gravitational field - the force exerted by the Earth on a sky diver in free fall, for example.
The sky diver experienced a force of 5 g. (not "5 g's")
g is measured in metres per second per second (m/s2) and has a value of 9.81 m/s2 on Earth.
Not to be confused with G.