D mesonn. Either of two mesons, one neutral and one positively charged, containing a charm quark and having masses 3,649 and 3,658 times that of an electron and mean lifetimes of about 4.2 × 10-13 and 1.04 × 10-12 second, respectively.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.