Wiktionary
rocket stage
n. A single rocket that may be attached to other similar rockets to increase the delta-v of the aggregated rocket vehicle
Usage examples of "rocket stage".
The MS-II, the big rocket stage on the back end of our Ares cluster, has performed flawlessly so far.
Before reaching the zenith of its flight, the rocket stage underneath the strange paraboloid payload falls away and tiny motors ignite on the underside of the flying boomerang.