"infection disease characterized by an abnormally high proportion of mononuclear leucocytes in the blood," 1917, coined from mononuclear + Modern Latin -osis "abnormal condition."

1959 as a shortening of monophonic in reference to recordings; earlier used among printers for "monotype machine" (c. 1925) and generally for monochrome (motorcar, etc.), 1940s. From 1964 as short for mononucleosis.