The Integrated Science Instrument Module (ISIM) is one of three major elements that comprise the Webb Observatory flight system. The others are the Optical Telescope Element (OTE) and the Spacecraft Element (Spacecraft Bus and Sunshield).
The ISIM is the heart of the James Webb Space Telescope, what engineers call the main payload. This is the unit that will will house the four main instruments that will detect light from distant stars and galaxies, and planets orbiting other stars. The structure (pictured left) is like a chassis in a car providing support for the engine and other components.
Mid-Infrared Instrument, or MIRI - provided by the European Consortium with the European Space Agency (ESA), and by the NASA Jet Propulsion Laboratory (JPL)