Meaning of carbon 12

carbon 12

Pronunciation: (kär'bun twelv'). [key]
  1. the isotopic carbon atom that comprises 99 percent of naturally occurring carbon, and that since 1961 has been used as the standard for atomic weight by representing a unit of 12.00000.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.