noun

1. Sound with a frequency greater than the upper limit of human hearing, which is approximately 20 kilohertz.

2. The use of ultrasonic waves for diagnostic or therapeutic purposes.

noun

1. A performer who understudies; a standby.

Synonyms: standby

adjective

1. Unusable; of no use.

2. Not working (machinery, etc).

3. Impractical.

Synonyms: u/s