In spacecraft design, the function of the thermal control system (TCS) is to keep all the spacecraft's component systems within acceptable temperature ranges during all mission phases. It must cope with the external environment, which can vary in a wide range as the spacecraft is exposed to deep space or to solar or planetary flux, and with ejecting to space the internal heat generated by the operation of the spacecraft itself.
Thermal control is essential to guarantee the optimal performance and success of the mission because if a component is subjected to temperatures which are too high or too low, it could be damaged or its performance could be severely affected. Thermal control is also necessary to keep specific components (such as optical sensors, atomic clocks, etc.) within a specified temperature stability requirement, to ensure that they perform as efficiently as possible.
In spacecraft design, a Sun shield restricts or reduces heat caused by sunlight hitting a spacecraft. An example of use of a thermal shield is on the Infrared Space Observatory. The ISO sunshield helped protect the cryostat from sunlight, and it was also covered with solar panels.
Not to be confused with the concept of a global-scale Sun shield in geoengineering, often called a Space sunshade or "Sun shield", in that case, the spacecraft itself is used to block sunlight on a planet, not as part the spacecraft's thermal design.
An example of a sunsheild in spacecraft design is the Sunshield (JWST) on the planned James Webb Space Telescope.