Definitions for "Beta rays"
a form of ionizing radiation emitted by radioactive substances (such as radium), more penetrating than alpha rays, and consisting of negatively charged electrons. The electrons are the same kind of particle as those of cathode rays, but have much higher velocities (about 35,000 to 180,000 miles per second). They are readily deflected by a magnetic or electric field.
historically, another name for electrons
High speed electrons.