Sound at frequencies above the audible range, namely above 20 kHz, audible only to various non-human species. Because of its very short WAVELENGTH in the megahertz range, ultrasound is used as a safe alternative to X-ray photography in medical diagnosis. Ultrasound scanning is sometimes called sonography.
SUPERSONIC was once used in acoustics synonymously with ultrasonic, but the former is now associated exclusively with speeds higher than that of sound.
See: AUDIO FREQUENCY, FREQUENCY, SONAR, SONICS. Compare: INFRASONIC, UHF.