The fixed rear element in the capacitor/diaphragm of a condenser (capacitor) microphone.
"I just wanted to say thank you for the work you recently did for me. I was actually doing this to help some old friends that don't have access to the web or really know how to go about finding these things out. You took an old home made 78 that was a recoding of his uncle from the late 40's. When I played it for them they were in tears since ..."