car·bon-14 (kär?b?n-fôr-t?n?)n. A naturally occurring, radioactive carbon isotope with an atomic mass of 14 and a half-life of 5,730 years, used in determining the age of ancient organic, geologic, or archaeological specimens.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.