[ pahy-on ]
/ ˈpaɪ ɒn /

noun Physics.

the first meson to be discovered: it has spin 0 and may be positively or negatively charged or neutral; charged pions decay into a muon and a neutrino or antineutrino. Symbol: π

Nearby words

  1. pinza,
  2. pinza, ezio,
  3. pinzón,
  4. pio,
  5. piolet,
  6. pioneer,
  7. pioneer day,
  8. pioneering,
  9. pioneers, the,
  10. piopio

Origin of pion

First recorded in 1950–55; pi (meson) + -on1

Also called pi meson.

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for pion

British Dictionary definitions for pion


pi meson

/ (ˈpaɪɒn) /


physics a meson having a positive or negative charge and a rest mass 273.13 times that of the electron, or no charge and a rest mass 264.14 times that of the electron

Word Origin for pion

C20: from Greek letter pi 1 + on

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for pion



1951, from Greek letter pi + -on.

Online Etymology Dictionary, © 2010 Douglas Harper

Science definitions for pion


[ pīŏn′ ]

A meson occurring either in a neutral form with a mass 264 times that of an electron and a mean lifetime of 8.4 X 10-17 seconds or in a positively or negatively charged form with a mass 273 times that of an electron and a mean lifetime of 2.6 X 10-8 seconds. The pion was once believed to be the particle that mediates the strong force, which holds nucleons together in the nucleus; it is now believed that the gluon is the mediator particle. Pions do interact with nucleons, however, and are able to transform neutrons into protons and vice versa. Also called pi-meson See Table at subatomic particle.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.