Dissipation in a thermodynamic system is an irreversible loss of ability to do work in a system; in other words, the energy gets converted into its forms which are inaccessible for work eventually due to the second law of thermodynamics. For example, dissipation of a mechanical energy to the heat absorbed by the environment via friction.
Often dissipation can be accounted for by working with non-Hermitean effective Hamiltonians.
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