1. Collective name for three semistable mesons which have charges of +1, 0, and -1 times the proton charge, and form a charge multiplet, with an approximate mass of 138 megaelectronvolts, spin 0, negative parity,negative G parity, and positive charge parity (for the neutral meson). Also known as pion. Symbolized π.

2. Any meson belonging to an isospin triplet with hypercharge 0, negative G parity, and positive charge parity (for the neutral meson).