The remote control of my CD player has a single button INDEX. I think that skips forward to the next index point but without a CD to test with I'm not sure. If I press it, the player appears to seek for several seconds, before starting again at the beginning of the track. So I guess an index is like a tape mark on magnetic tape.

I have several CDs where a track comprises several tunes run together (an example is in this thread.) It might be (slightly) useful in such cases.