Ultrasonic means "at a sound pitch too high for human ears". A young person can hear up to roughly 20,000 vibrations a second; anything faster than that is ultrasonic. The key idea to hold on to is that ultrasonic sound is still real sound — it moves through air and water and bodies just like the sound you hear. Only your ear fails; the sound does not stop existing. That is why animals matter here: dogs, bats, mice and dolphins hear well above our limit, so a "silent" dog whistle is silent only to you. And it is why the word turns up in so many machines. Ultrasonic waves bounce off things and come back, so we use them to see inside the body (that is the same technology as a pregnancy scan), to measure distance in a car's parking sensors, and to clean glasses and jewellery in a little tank, where the fast vibrations shake the dirt loose. It is an adjective, used before a noun: ultrasonic waves, an ultrasonic sensor, ultrasonic cleaning. There is an adverb, ultrasonically, but it is rare and technical.