Background radiation - radiation from natural sources

Bar magnet - a rectangular piece of iron that has a magnetic field

Baryon - a particle made up of three quarks

Baryon number - a conserved quantum number j it is assigned to each quark and by extension to baryons

Battery - a source of emf

Best estimate - the average value of a set of measurements of a given quantity that will serve as the quoted value for that quantity

Beta minus decay - a decay producing an electron and an anti-neutrino

Beta particle - an electron

Beta plus decay - a decay producing a positron and a neutrino

Binding energy - the minimum energy that must be supplied to completely separate the nucleons in a nucleus or the energy released when a nucleus is assembled