Gamma ray:  A high energy photon. 
Graham's law of diffusion:  The rate of diffusion of a gas is inversely proportional to the square root of its density. 
Gravitational constant (G):  The constant of proportionality in Newton's law of gravitation, G = 6.67 x 1011 Nm2/kg2 
Gravitational potential at a point:  The amount of work done against the gravitational forces to move a particle of unit mass from infinity to that point. 
Gravitational potential energy:  The energy possessed by a body due to its position. 
Gravity:  The gravitational attraction at the surface of a planet or other celestial body. 
Ground state:  The lowest energy state of an atom. 
