The distance between the position of a moving object such as a speaker diaphragm, and its original position.
"I wanted to thank you for the wonderful work that you did on the 5 home-recorded 78 records that I sent you. These were made by my Grandfather in the late 1940's and 1950's. No one in the family had heard them in almost 50 years. They varied in what shape they were in, and we really thought that they would not be playable after so many years. ..."