Waste heat is the by-product heat of machines and technical processes for which no useful application is found.

A fraction of input energy is always converted to heat by friction between machine parts and other processes such as liquid friction (see: viscosity).

Mechanical drives can be designed to run smoothly, with little loss of energy to heat. Machines for conversion of energy contained in fuels to mechanical work or electric energy produce large amounts of waste heat (see: Second law of thermodynamics).