Definition of safety factor

Magnetic field lines on closed magnetic surfaces travel closed curves on the poloidal plane. Safety factor $q$ is defined as the number of toroidal loops a magnetic field line goes when it makes one poloidal loop, i.e.

$$q \equiv \frac{\triangle \phi}{2 \pi},$$ (25)

where $\triangle \phi$ as the change of the toroidal angle when a magnetic field line travels a full poloidal loop.