A mo-2-on (pronounced without vocalizing the "2", so with a glottal stop, like "mutton" but with an "o") is a celestial object that orbits as a moon of a moon. None have yet been observed.
Kollmeier, Juna A.; Raymond, Sean N. (2018-10-09). "Can Moons have moons?" (PDF). ArXiv. Retrieved 2018-10-10.