A line in an electric or magnetic field that shows the direction of the force.

The lines that can be produced by a permanent magnet or a use of a sheet of paper and iron filings are called "magnetic lines of force". These lines are imaginary lines, but the concept is convenient for describing a magnetic field. A magnetic field has an intensity of so may lines per square inch. These lines have a definite direction and always form closed loops and return back onto themselves.

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

a line drawn through a magnetic field in the, direction of the force at each point through which it passes

an imaginary line drawn in such a way that its direction at any point is the same as the direction of the field at that point

Line of force or line of flux , usually taken in the context of electromagnetism, is the curve whose tangent gives the direction of the field at that point. As a result, it also runs perpendicular to the equipotential lines in the conventional direction from higher to lower potential.