B0

In MRI, B0 refers to the strong primary field used to polarize the protons in the body.

Bending magnet

In accelerators, a bending magnet is a dipole magnet used to redirect the particle beam. 

B/H

Ferromagnetic materials are characterized by measuring their hysteresis loop, or B/H loop. H is the applied field, and B is the field in the material.

Bore

The centre of a solenoidal magnet, where the magnetic field is concentrated.

Bucking coil

Bucking coils are usually used in rotating-coil systems for characterizing accelerator magnets. They subtract out part of the response of the main coil, permitting a direct and precise measure of a field gradient.