Friction is a lumped label for energy dumping to outside systems.

Evidence

Any real oscillator can temporarily confine energy given to it, but is always coupled to something else. We need to include the effects of dumping of the vibrational energy to these other things. If there are myriad “other things” to accept the energy, and not give it back, we concentrate on the system and lump all the energy dumping into something we call friction. — Eric J. Heller, Why You Hear What You Hear- An…, 938

Categorization

• DEFINE: Presents the conceptual move of lumping all energy loss out of a system into a single ‘friction’ term.