MU Meaning of MU in English /myooh, mooh/ , n.1. the 12th letter of the Greek alphabet (M, µ). 2. the consonant sound represented by this letter. 3. micron (def. 1). [ 1895-1900; my ] Random House Webster's Unabridged English dictionary. Полный английский словарь Вебстер - Random House . 2012