The stars and gas clouds in the disk of a spiral galaxy have circular orbits within the plane of the disk. It is sometimes convenient to picture the disk of a galaxy as a disk, using a cylindrical coordinate system. The height above or below the disk is usually referred to as the "z height". When z = 0, the object lies exactly in the midplane of the disk.
Stars and gas clouds in the disk move primarily in their circular orbits, but they may bob up and down slightly within the disk (much like the horses on a merry-go-round). This motion is to be compared with the random motions seen in the spheroidal components.
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