A cylinder is basically the set of all tracks that all the heads are currently located at. If a disk has four platters, in general case it would have eight heads. Now suppose it has cylinders number 720.
It would be made up of the eight set of tracks, one per platter surface with tracks number 720. The name comes from the fact that these tracks form a skeletal cylinder because they are equal-sized circles stacked one on top of the other in space, as shown in the figure given before.
The addressing of the factors of the disk is traditionally done by referring to cylinders, heads and sectors (CHS).