A physical hole or notch, or a recorded code or mark, that is used to identify a starting point for each track on a disk.
"index mark." YourDictionary, n.d. Web. 15 August 2018. <http://www.yourdictionary.com/index-mark>.
index mark. (n.d.). Retrieved August 15th, 2018, from http://www.yourdictionary.com/index-mark