B mesonn. Either of two mesons, one neutral and one positively charged, containing a bottom antiquark and having masses about 10,330 times that of an electron and mean lifetimes of about 1.6 × 10-12 second.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.