Definition of micromillimetre in English:
micromillimetre
(also micromillimeter)
noun ˈmʌɪkrə(ʊ)ˌmɪlɪmiːtə
1Chiefly Histology. One thousandth of a millimetre, a micron or micrometre. Now rare.
2In general use: an extremely tiny distance.
Origin
Mid 19th century. From micro- + millimetre.