hypothetical, very light, weakly interacting boson
proposed in the late 1970s (Peccei and Quinn, Weinberg, Wilczek) to solve the strong CP problem: why the CP-violating parameter of QCD is below ~10⁻¹⁰ (from the neutron electric dipole moment limit)
dark matter candidate
search: axion → photon conversion in a strong magnetic field (e.g. ADMX microwave cavity, scanning one mass step at a time)