Dictionary.net

That part of the science of acoustics dealing with the frequency range beyond the upper limit of perception by the human ear (beyond 20,000 hertz per second), but usually restricted to frequencies above 500,000 hertz per second. Ultrasonic radiation is injurious to tissues because of its thermal effects when absorbed by living matter, but in controlled doses it is used therapeutically to selectively break down tissues, as in treatment of arthritis and lesions of the nervous system, and also as a diagnostic aid by visually displaying echoes received from irradiated tissues, as in echocardiography and echoencephalography. (Dorland, 28th ed)