microunit
/ˈmʌɪkrə(ʊ)juːnɪt/Chiefly Medicine
noun
A millionth part of a unit of measurement; specifically one millionth of an international unit (as of insulin or other hormone).
Origin
Early 20th century; earliest use found in William Dorland (1864–1956), obstetrician. From micro- + unit adjective.