Dissipation is the result of irreversible processes that take place in inhomogeneous systems. These processes produce entropy (see entropy production) at a certain rate. The entropy production rate times ambient temperature gives the dissipated power. Important examples of irreversible processes are: heat flow through a thermal resistance, fluid flow through a flow resistance, diffusion (mixing), chemical reactions, and electrical current flow through an electrical resistance (Joule heating). The concept of dissipation was introduced in the field of thermodynamics by William Thomson (Lord Kelvin) in 1852.
Dissipating forces are those that can not be described by Hamiltonian formalism. Loosely speaking, this includes friction, and all similar forces that result in decoherency of energy—that is, conversion of coherent or directed energy flow into an indirected or more isotropic distribution of energy.