mu / m(y)oō/ • n. the twelfth letter of the Greek alphabet (Μ, μ), transliterated as ‘m.’ ∎  (Mu) [followed by Latin genitive] Astron. the twelfth star in a constellation: Mu Cassiopeiae. ∎  [as adj.] Physics relating to muons: mu particle.• symb. ∎  (μ) micron. ∎  (μ) [in comb.] “micro-” in symbols for units: the recommended daily amount is 750μg. ∎  (μ) permeability.

Mu (Jap.; Chin., wu). Zen emptiness of content, nothingness, closely related to śūnyatā. From this arises the first kōan of the Wu-men-kuan (Mumonkan), which introduces the Zen student to ‘the world of mu’: ‘A monk asked master Chao-chou respectfully: “Does a dog actually have a Buddha-nature or not?” He replied: “Mu”’. The opposite is U.