Transformation of one form of energy into another.
In Latin, dissipation literally means “scattering”; in physics, it refers to a process in a dynamic system during which due to e.g. friction the energy of structured macroscopic motion is transformed into thermal energy, i.e., into the energy of unstructured molecular motion. This type of system is dubbed dissipative. The term is common in branches of physics like thermodynamics and acoustics, and in wave theory in general.
In thermodynamics, when work is transformed into thermal energy (internal energy) due to friction, compression or isochoric processes, these are referred to as dissipative processes.