line of force

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an imaginary line in a field of force; direction of the line at any point is the direction of the force at that point