/ Dictionary / Index A alpha particle: Meaning and Definition of Find definitions for: al'pha par"ticle Pronunciation: [key] — Physics. Physics. a positively charged particle consisting of two protons and two neutrons, emitted in radioactive decay or nuclear fission; the nucleus of a helium atom. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. alphanumeric alpha-particle scattering Related Content Daily Word Quiz: loathe Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language