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Dissipation, in the context of physical systems, is the irreversible transformation of energy into less usable forms, most often heat, due to interaction with the surroundings or internal friction. In a complete, isolated system total energy remains constant, but the energy carried by a subsystem can decrease as it dissipates into the environment, accompanied by an increase in entropy in line with the second law of thermodynamics.

Dissipation arises from non-conservative forces in mechanics, viscous effects in fluids, electrical resistance in circuits, and radiative losses in optics. In mechanical modeling, damping represents forces that remove kinetic energy from motion. In fluids, viscous dissipation converts kinetic energy of the flow into internal energy, heating the fluid. In circuits, resistance converts electrical energy into heat (Joule heating). Dissipation can be essential for stabilizing systems (as in dampers) or undesirable when energy loss is wasteful.

In quantum mechanics, dissipation is treated through open-system approaches, where a system exchanges energy with an environment, leading to decoherence and energy relaxation. In engineering and applied sciences, understanding and managing dissipation is important for design and analysis, influencing everything from vibration control and insulation to energy harvesting and signal attenuation. The study of dissipation also relates to irreversibility and the formation of dissipative structures in nonlinear dynamical systems.