A pion (π) is a type of hadron,
i.e., a type of a composite particle made up of quarks.
More specifically, it is a type of meson, a hadron
made of exactly two quarks.
In contrast, protons and neutrons are hadrons made up of three quarks
each.
Pions decay extremely rapidly (into multiple leptons
or photons), but are involved
in the strong force within atomic nuclei, and are relevant
in some of the extreme circumstances found in astrophysics,
e.g., the interior of neutron stars (pion condensates),
and the unfolding of the veryearly universe.