1More fully "International System of Units". A system of physical units having as base units the metre, kilogram, second, ampere, kelvin, candela, and mole, with derived units defined in terms of these by a set of prefixes indicating multiplication or division by a power of ten; abbreviated SI.
The system is based on the metric system, but with refined definitions avoiding as far as possible reliance on a physical artefact as the defining object. Following the redefinition of the metre the only unit now relying on such a standard is the kilogram.
A system of dimensions and proportions for screw threads for screws and bolts of various sizes, based on metric units of measurement. Now chiefly historical.
First proposed, as a system for standardizing screw threads across Europe, in 1898 at the Congrès international pour l'unification des filetages.
International System/ˌɪntəˈnaʃ(ə)n(ə)l ˌsɪstɪm/ /ˌɪntəˈnaʃən(ə)l ˌsɪstɪm/
Mid 19th century (in an earlier sense). From international + system.