Imaginary magnetic lines used as a means of explaining the behaviour of magnetic fields. Their conception is based on the pattern of lines produced when iron filings are sprinkled on a piece of paper laid over a magnet. Synonymous with Magnetic Lines of Force.
Source: API RP 5A5, Field Inspection of New Casing, Tubing, and Plain-end Drill Pipe, Reaffirmed August 2010. Global Standards