The portion of the energy input to a mechanical process that is rejected into the environment: There is something ironic about air conditioners that cool a person's home but which generate a lot of waste heat which is poured out into the environment.

Any system or process that actively captures warmth which would otherwise be discharged into the environment, so that it can be used for other purposes: The use of the heat from exhaust gases to heat water is an example of waste heat recovery.