Radiation refers to:

Radiation refers to the transmission of energy in the form of electromagnetic waves or particles. This process does not require a medium, which distinguishes it from other forms of heat transfer such as conduction and convection. In radiation, energy can travel through a vacuum, as is the case with heat from the sun reaching the Earth.

The concept is central to understanding how heat is transferred in various systems, particularly in power engineering contexts, where the management of energy in different forms is crucial. For instance, in nuclear power plants, radiation is a significant aspect to consider for safety and efficiency.

The other options describe different processes: mechanical energy transmission relates to kinetic energy moving through a medium, heat transfer from molecule to molecule is how conduction operates, and the transfer of heat by different densities describes convection, where warmer, less dense fluids rise while cooler, denser fluids sink. These processes are distinct from radiation, which operates through the emission and absorption of electromagnetic waves.