  We show that the mass and width of an unstable particle are precisely defined
by the pole in the complex energy plane, $\mu = m - (i/2)\Gamma$, by
identifying the width, $\Gamma$, with the particle's decay rate and the mass,
$m$, with the oscillatory frequency. We find that the physical $Z$ boson mass
lies 26 MeV below its quoted value, while the physical $W$ boson mass lies 20
MeV below. We also clarify the various Breit-Wigner formulae that are used to
describe a resonance.
