A frequency that is above the range of human hearing. Conventionally, this is given as 20 KHz, but few adults can actually hear that high; 15-18 KHz is more typical, and some adults who have worked in certain occupations in which they are exposed to certain types of noise may not hear much above 10 KHz.
Some synth circuits can generate ultrasonics, usually as a result of processing such as distortion or ring modulation. Ultrasonics can be insidious because although humans cannot hear them, they can cause distress to human and animal ears, and they can also burn out voice coils in loudspeakers.
The opposite of ultrasonic is subsonic.