Beta particle

(Or β particle.) Physically indistinguishable from the electron (or positron) but usually restricted to products in nuclear reactions (beta decay).

The term was coined by Ernest Rutherford, who discovered that the ionizing radiation emitted by uranium consisted of "at least two distinct types . . . one that is very readily absorbed . . . the α radiation, and the other of a more penetrative character . . . the β radiation." Kinetic energies of beta particles range from tens of thousands to millions of electron volts. Because either electrons or positrons are emitted in beta decay, the term beta particle, a relic of an era in which its identity was unknown, is falling into disuse.
See alpha particle, gamma ray.
Boorse, H. A., and L. Motz 1966. The World of the Atom. Vol. I, . 437–445.

An aggregation of sufficiently many atoms or molecules that it can be assigned macroscopic properties such as volume, density, pressure, and temperature.

The energy that a body possesses as a consequence of its motion, defined as one- half the product of its mass and the square of its speed, (1/2)m'v².