Dictionary.com
definitions

alpha particle

noun Physics.
  1. a positively charged particle consisting of two protons and two neutrons, emitted in radioactive decay or nuclear fission; the nucleus of a helium atom.

Origin of alpha particle

First recorded in 1900–05
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for alpha particle

alpha particle

noun
  1. a helium-4 nucleus, containing two neutrons and two protons, emitted during some radioactive transformations
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
alpha particle in Medicine

alpha particle

n.
  1. A positively charged nuclear particle, indistinguishable from a helium atom nucleus and consisting of two protons and two neutrons.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
alpha particle in Science

alpha particle

  1. A positively charged particle that consists of two protons and two neutrons bound together. It is emitted by an atomic nucleus undergoing radioactive decay and is identical to the nucleus of a helium atom. Because of their relatively large mass, alpha particles are the slowest and least penetrating forms of nuclear radiation. They can be stopped by a piece of paper. See more at radioactive decay.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.