Dictionarypi•onPronunciation: (pī'on), [key] —n. 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 Also called pi meson. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.
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