MOSAIC is the Multi-Object Spectrograph (MOS) for the 39m Extremely Large Telescope (ELT) of the European Southern Observatory (ESO), with unique capabilities in terms of multiplex, wavelength coverage and spectral resolution.It is a versatile multi-object spectrograph working in both the Visible and NIR domains, designed to cover the largest possible area (∼40 arcmin2) on the focal plane, and optimized to achieve the best possible signal-to-noise ratio on the faintest sources, from stars in our Galaxy to galaxies at the epoch of the reionization.In this paper we describe the main characteristics of the instrument, including its expected performance in the different observing modes.The status of the project will be briefly presented, together with the positioning of the instrument in the landscape of the ELT instrumentation.We also review the main expected scientific contributions of MOSAIC, focusing on the synergies between this instrument and other major ground-based and space facilities.