Transient

In acoustics and audio, a transient is a short-duration signal that represents a non-harmonic attack phase of a musical sound or spoken word. It contains a high degree of non-periodic components and a higher magnitude of high frequencies than the harmonic content of that sound. Transients do not directly depend on the frequency of the tone they initiate. See also onset (audio).

Sonar
Transient is often used by sonar operators to enter large numbers of crit strings, referring to sudden, unnatural changes in the acoustic environment. This is usually caused by an unnaturally fast moving object. It is most often applied for military use, referring to unexpected sounds emanating from another vessel such as operating machinery, a metal hatch being slammed, or the flooding and pressurization of torpedo or vertical launch tubes