Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing.
Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound.
The production of ultrasound is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy.
The reflection signature can reveal details about the inner structure of the medium.
The most well known application of this technique is its use in sonography to produce pictures of fetuses in the human womb. There are a vast number of other applications as well.
See Wikipedia, Ultrasound.