Also found in: Encyclopedia, Wikipedia.
An electrically neutral meson having a mass 1,071 times that of the electron and a mean lifetime of about 5 × 10-19 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.