The acceleration produced by the mutual attraction of two masses, directed along the line joining their centers of masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass.
This acceleration on a unit mass has the magnitude
where m is the mass of the attracting body, r is the distance between the centers of mass, and G is the gravitational constant equal to 6.670 +/- 0.005 X 10-8 cm2/gram sec2. In the case of masses in the earth's gravitational field, m is the mass of the earth, equal to 5.975 X 1027 grams. However, the rotation of the earth and atmosphere modifies this field to produce the field of gravity.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use