proton

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The stable, positively charged nucleon, having a mass 1,836 times that of an electron and being a baryon composed of two up quarks and one down quark. The proton is a basic component of all atomic nuclei and the nucleus of the protium isotope of hydrogen.

from The Century Dictionary.

• noun The first inception or rudiment of a structure or organ in the embryo: a term suggested as an equivalent of the German anlage.
• noun A term designating certain products of tryptie digestion which are obtained from the protamins and which, supposedly, are intermediary between these and the end-products.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element; the nucleus of the most common isotope of hydrogen; composed of two up quarks and a down quark

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a stable particle with positive charge equal to the negative charge of an electron