The region in which the magnetic forces created by a permanent magnet or by a current-carrying conductor or coil can be detected.
These are invisible lines of force which surround all magnets and which pass from the north pole to the south pole of the magnet.
Magnetic field lines around a permanent magnet.
The magnetic field lines can be visualised using iron filings or a compass.
Note the alignment of a compass with respect to the magnetic field lines.