Magnetic flux density B

The flux density is the number of magnetic field lines that pass through a certain point on a surface. The SI unit is T (tesla), which is equal to weber per square metre (Wb/m²). The unit in the CGS system is G (gauss). 1 tesla is equal to 10,000 gauss.

At any given point in a magnetic field, you can see the magnetic flux density as a vector in the field direction with a magnitude equal to the Lorentz force that an electrical wire experiences when it is oriented perpendicular to the field lines.

The higher the flux density, the stronger the magnet is at that point and thus the better it can hold iron particles at that point.