chi meson (k?, k?)n. An electrically neutral meson composed of a charm quark and charm antiquark and having a mass 6,683 times that of an electron and a mean lifetime of about 6.3 × 10-23 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.