A field is a mathematical construct to help visualize the effects of forces. The lines of the field indicate the direction a particle will move (be forced) if dropped into the field at that point. The field concept plays a central role in the classical formulation of electromagnetism, as well as in many other areas of classical and contemporary physics. Einstein's gravitational field, for example, replaces Newton's concept of gravitational action at a distance. The field describing the electric force between a pair of charged particles works in the following manner: each particle creates an electric field in the space surrounding it, and so also at the position occupied by the other particle; each particle responds to the force exerted upon it by the electric field at its own position.

Excerpt from the Encyclopedia Britannica without permission.