In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons (and, more generally, the lightest hadrons), because they are composed of the lightest quarks (the u and d quarks). They are unstable, with the charged pions and decaying with a mean lifetime of 26 nanoseconds ( seconds), and the neutral pion decaying with a much shorter lifetime of seconds. Charged pions most often decay into muons and muon neutrinos, and neutral pions into gamma rays.

Protein pigeon homolog also known as gamma-secretase activating protein (GSAP) is a protein that in humans is encoded by the PION gene.