Gravitational Field
We say that a body A exerts a force of gravitation on another body B kept at a distance. This is called action at  a distance viewpoint. However, this viewpoint creates certain problems when one deals with objects separated by large distances. It is now assumed that a body can not directly interact with another body kept at a distance. The force between two objects is seen to be two-step process.
Int he first step, it is assumed that the body A creates a gravitational field in the space around it. The field has its own existence and has energy and momentum. his field has a definite direction at each point of the space and its intensity varies from point to point.
In the second step, it is assumed that when a body B is placed in a gravitational field, this field exerts a force on it. The direction and the intensity of the field is defined in temrs of the force it exerts on a body placed in it. We define the intensity of gravitational field E at a point by the equation.
E = F/m
where F is the force exerted by the field on a body of mass m placed in the field. Quiet often the intensity of gravitational field is abbreviated as gravitational field. Its SI unit is N/kg