line of force

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A theoretical line in a field of force, such that a tangent at any point gives the direction of the field at that point.

Wiktionary

1. n. physics an imaginary line in a field of force, whose tangent at any point gives the direction of the field at that point and whose number through unit area perpendicular to the field represents its intensity

GNU Webster's 1913

1. n. (Physics) any line in a space in which forces are acting, so drawn that at every point of the line its tangent is the direction of the resultant of all the forces. It cuts at right angles every equipotential surface which it meets. Specifically (Magnetism), a line in proximity to a magnet so drawn that any point in it is tangential with the direction of a short compass needle held at that point.

WordNet 3.0

1. n. an imaginary line in a field of force; direction of the line at any point is the direction of the force at that point