Webster's English Dictionary

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proton

pro.ton \'pro--.ta:n\ \pro--'ta:n-ik\ n [Gk pro-ton, neut. of pro-tos first 
   - more at PROT-] : an elementary particle that is identical with the 
   nucleus of the hydroge n atom, that along with neurons is a constituent of 
   all other atomic nuclei, that carries a positive charge numerically equal 
   to the charge of an electron, and that has a mass of 1.672X10-24 gram - 
   pro.ton.ic aj