What is defined as a closed heating appliance that supplies hot water or steam for various purposes?

The definition of a closed heating appliance that supplies hot water or steam for various purposes is accurately described by a boiler. A boiler operates by heating water to produce steam or hot water, which can then be distributed through a building for heating, sterilization, or other processes. The closed nature of the system allows for the efficient circulation of water or steam while maintaining pressure and temperature.

This is in contrast to water heaters, which are generally focused on providing hot water for domestic use without the same emphasis on generating steam or functioning in high-pressure applications. Heat exchangers and radiators serve different roles in a heating system; a heat exchanger transfers heat between fluids, and a radiator distributes heat into a space rather than generating it. Thus, the characteristics and functions of a boiler align perfectly with the definition provided in the question.