Source : Webster's Revised Unabridged Dictionary (1913)

Beta rays \Be"ta rays\ (Physics)
   Penetrating rays readily deflected by a magnetic or electric
   field, emitted by radioactive substances, as radium. They
   consist of negatively charged particles or electrons,
   apparently the same in kind as those of the cathode rays, but
   having much higher velocities (about 35,000 to 180,000 miles
   per second).