noun Physics, Chemistry.
a positively charged elementary particle that is a fundamental constituent of all atomic nuclei. It is the lightest and most stable baryon, having a charge equal in magnitude to that of the electron, a spin of ½, and a mass of 1.673 × 10− 27 kg. Symbol: P

1915–20; noun use of Greek prôton, neuter of prôtos first

protonic, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
proton (ˈprəʊtɒn)
a stable, positively charged elementary particle, found in atomic nuclei in numbers equal to the atomic number of the element. It is a baryon with a charge of 1.602176462 × 10--19 coulomb, a rest mass of 1.672 62159 × 10--27 kilogram, and spin ½
[C20: from Greek prōtos first]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1920, coined by Eng. physicist Ernest Rutherford (1871-1937) from Gk. proton, neut. of protos "first," supposedly because hydrogen was hypothesized as a constituent of all the elements. The word was used earlier in embryology (1893) at a transl. of Ger. anlage ("fundamental thing") based on Aristotle's
phrase he prote ousia to proton.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

proton pro·ton (prō'tŏn')
A stable, positively charged subatomic particle in the baryon family having a mass 1,836 times that of the electron.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
proton   (prō'tŏn')  Pronunciation Key 
A stable subatomic particle in the baryon family having a mass of 1.672 × 10-24 grams (1,836 times that of the electron) and a positive electric charge of approximately 1.602 × 10-19 coulombs. Protons make up part of the nucleus of all atoms except hydrogen, whose nucleus consists of a single proton. In neutral atoms, the number of protons is the same as the number of electrons. In positively charged atoms, the number of protons is greater than the number of electrons, and in negatively charged atoms electrons outnumber protons. Protons are believed to be composed of two up quarks and one down quark. See Table at subatomic particle.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary
proton [(proh-ton)]

An elementary particle with a positive charge, found in the nucleus of an atom.

Note: A proton is over a thousand times heavier than an electron.
Note: Protons and neutrons make up most of an atom's mass.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Computing Dictionary

PROTON definition

1. A home computer made by Acorn Computers under a contract won from the BBC in April 1981.
2. Something to do with Microsoft SoftLib?
The Free On-line Dictionary of Computing, © Denis Howe 2010
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Example sentences
The pull appeared as strong as an electron toward a proton.
Because the quarks and gluons within a proton are constantly interacting, a
  proton-proton collision is an inherently messy affair.
For example, a proton has a positive electric charge, but an antiproton has a
  negative electric charge.
The molecules are composed of atoms, which in turn are composed of electrons
  and clusters of proton particles.
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