Merriam-Webster Collegiate Dictionary - audio

 I. adjective  Etymology: ~-  Date: 1916  1. of or relating to acoustic, mechanical, or electrical frequencies corresponding to normally audible sound waves which are of frequencies approximately from 15 to 20,000 hertz  2.  a. of or relating to sound or its reproduction and especially high-fidelity reproduction  b. relating to or used in the transmission or reception of sound — compare video  c. of, relating to, or utilizing recorded sound  II. noun  Date: 1934  1. an ~ signal; broadly sound  2. the section of television or motion-picture equipment that deals with sound  3. the transmission, reception, or reproduction of sound