A signal wave, usually a single frequency, transmitted over the system to indicate or control its characteristics.
"Thanks for your work creating a CD for me! I had your company make a CD from an old record for me about 8 months ago. Because it was a Christmas album, I didn't play it until Christmas, which is why I didn't write anything until now. Your did a great job making the CD. It sounds as good as I remember the old record sounding. It is wonderful to ..."