Definition of "axion"

The axion is a candidate particle for the cold dark matter that constitutes a large fraction of the mass of the Universe.

2000 Summer/Fall, John Clarke, Superconductivity: A Macroscopic Quantum Phenomenon, Rene Donaldson, Bill Kirk (editors), Beam Line, Volume 30, Number 2, Stanford Linear Accelerator Center, page 47

In summary, the first version of gravity-mediated supersymmetry breaking has the advantage of giving an axion mass that is within cosmological bounds, while the second version has the advantage of giving the gauginos masses that are comparable to the masses of the squarks and sleptons.

2000, Steven Weinberg, The Quantum Theory of Fields, volume 3, Supersymmetry, page 375

Let us begin with a disclaimer: axions, a, have been conjectured to exist on theoretical grounds, but have not as yet been observed. They are supposed to be very light particles that interact very weakly with matter.

2013, Adrian C. Melissinos, Reminiscences: A Journey Through Particle Physics, page 93

Theoretical considerations appear to indicate that the axion mass should be somewhere between the microelectron volt and millielectron volt range.

2015, Paul Seidel, editor, Applied Superconductivity: Handbook on Devices and Applications, page 857