The application of currents of low tension and high amperage, that produce warmth in the deeper parts of the body; also known as, medical diathermy: "Diathermy", or thermopenetration, is the heating of the bodily tissues resulting from their resistance to the passage of high-frequency electromagnetic radiation, electric currents, or ultrasonic waves.

In medical thermopenetration, the tissues are heated but not damaged; however, in surgical diathermy (electrocoagulation) tissue is destroyed because of the use of a high-frequency electric current to bring about the coagulation and destruction of such tissue.