The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
An electrically neutral baryon having a mass 11,000 times that of the electron and a mean lifetime of approximately 1.1 × 10-12 seconds. See Table at subatomic particle.